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Maybe I am just talking out my ass but why must they make it so difficult to identify by making you to turn off the main water and removing the part to find the number or taking it to Home Depot to match when they can just simply put a model number on the stem end for anyone to see??? It would be so much easier to order the part or at least check inventory status for the closest supply store before having to remove the whole cartridge.
I'm confused, you have two cartridges in your hand and all the cartridges are in place in the shower valve. Is it a different shower?
If there are no markings anywhere on the valves or handles, You've got two choices: go to your home store and look on the plumbing racks for an identical cartridge (bring the old one with) or go to a plumbing supply store and they should be able to match it. Those guys usually know their stuff. It will be hard to find one online without actually being able to line it up with the old one. Good luck and stay safe out there.
Di Vapor Victoria Plum series
https://www.divapor.com/spares/showers/diverter-cartridge/100mm-two-way-diverter-cartridge-for-victoria-plum-shower-valves.php