Help Identifying this Shower Faucet 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.