This is a bit convoluted so bear with me. In _ctypes.c (version 3.8.5), there is on line 5788:

    Py_TYPE(&Simple_Type) = &PyCSimpleType_Type;
    Simple_Type.tp_base = &PyCData_Type;
    if (PyType_Ready(&Simple_Type) < 0)
        return NULL;
    Py_INCREF(&Simple_Type);
    PyModule_AddObject(m, "_SimpleCData", (PyObject *)&Simple_Type);

This exposes the address of Simple_Type as the name _SimpleCData. Simple_Type (a.k.a. _SimpleCData to the Python world) also inherits from PyCDataType (more on that later). First, the definition of Simple_Type is on line 5040:

static PyTypeObject Simple_Type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "_ctypes._SimpleCData",
    sizeof(CDataObject),                        /* tp_basicsize */
    0,                                          /* tp_itemsize */
    0,                                          /* tp_dealloc */
    0,                                          /* tp_vectorcall_offset */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_as_async */
    (reprfunc)&Simple_repr,                     /* tp_repr */
    &Simple_as_number,                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    0,                                          /* tp_getattro */
    0,                                          /* tp_setattro */
    &PyCData_as_buffer,                         /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /* tp_flags */
    "XXX to be provided",                       /* tp_doc */
    (traverseproc)PyCData_traverse,             /* tp_traverse */
    (inquiry)PyCData_clear,                     /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    Simple_methods,                             /* tp_methods */
    0,                                          /* tp_members */
    Simple_getsets,                             /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    0,                                          /* tp_descr_get */
    0,                                          /* tp_descr_set */
    0,                                          /* tp_dictoffset */
    (initproc)Simple_init,                      /* tp_init */
    0,                                          /* tp_alloc */
    GenericPyCData_new,                         /* tp_new */
    0,                                          /* tp_free */
};

This is in essence a metaclass used to define new classes. One of the members of this structure (with the comment /* tp_methods */) references the following array defined on line 4996:

static PyMethodDef Simple_methods[] = {
    { "__ctypes_from_outparam__", Simple_from_outparm, METH_NOARGS, },
    { NULL, NULL },

This should be a list of methods that classes created by this metaclass will have. But wait! That is only a single method. Let's look at the class it inherits from, PyCSimpleType_Type on line 2327:

PyTypeObject PyCSimpleType_Type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "_ctypes.PyCSimpleType",                                    /* tp_name */
    0,                                          /* tp_basicsize */
    0,                                          /* tp_itemsize */
    0,                                          /* tp_dealloc */
    0,                                          /* tp_vectorcall_offset */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_as_async */
    0,                                          /* tp_repr */
    0,                                          /* tp_as_number */
    &CDataType_as_sequence,             /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    0,                                          /* tp_getattro */
    0,                                          /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /* tp_flags */
    "metatype for the PyCSimpleType Objects",           /* tp_doc */
    0,                                          /* tp_traverse */
    0,                                          /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    PyCSimpleType_methods,                      /* tp_methods */
    0,                                          /* tp_members */
    0,                                          /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    0,                                          /* tp_descr_get */
    0,                                          /* tp_descr_set */
    0,                                          /* tp_dictoffset */
    0,                                          /* tp_init */
    0,                                          /* tp_alloc */
    PyCSimpleType_new,                                  /* tp_new */
    0,                                          /* tp_free */
};

The methods it brings along are PyCSimpleType_methods defined on line 2318:

static PyMethodDef PyCSimpleType_methods[] = {
    { "from_param", PyCSimpleType_from_param, METH_O, from_param_doc },
    { "from_address", CDataType_from_address, METH_O, from_address_doc },
    { "from_buffer", CDataType_from_buffer, METH_VARARGS, from_buffer_doc, },
    { "from_buffer_copy", CDataType_from_buffer_copy, METH_VARARGS, from_buffer_copy_doc, },
    { "in_dll", CDataType_in_dll, METH_VARARGS, in_dll_doc},
    { NULL, NULL },
};

So it appears that the mataclass builds a new class including all of the methods specified by the tp_methods member of the metaclass's struct definition and its base classes. This should result in the from_buffer method having the same address for all objects for all classes created by the metaclass.

Referring back to your post:

>>> ctypes.c_long.from_buffer
<built-in method from_buffer of _ctypes.PyCSimpleType object at 0x16f8fb0>
>>> ctypes._SimpleCData.from_buffer
<built-in method from_buffer of _ctypes.PyCSimpleType object at 0x7f19f9b09ae0>

I now believe you are misinterpreting what is being output. 0x16f8fb0 and 0x7f19f9b09ae0 are not the addresses of the from_buffer methods, which should be the same for both objects, but rather the addresses of the ctypes.PyCSimpleType objects themselves that are implementing these types.

Mark Tolonen had it right, but in case you needed convincing (I did too) ...