Shower diverter:
Need help identifying shower diverter cartridge for replacement
Plumbing – Need help identifying shower diverter cartridge
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Related Solutions
This is, without a doubt, a Moen brand valve. There are numerous trim options available in many finishes. You need to go to a good specialty plumbing shop, they will have some Moen trim items in stock and should be able to order many more items.
Bring your pictures and all old trim with you.
This looks like you have a Single Valve Control Shower .. where yours has temperature settings on it. Maybe this is your tempering valve, it can also be a Pressure Balance Valve or what sometimes can be referred to as anti-scalding valves these valves prevent burns from water when say the toilet is flushed - all the cold water flows to the toilet and that leaves only hot for the shower hence you get scorched.
More and better pictures will be needed if you want to know brand and model information.
BTW - the valve in the back might be part of the assembly and actually have the manufacturer model or info on it.
Best Answer
It's going to be difficult identifying that part without a model number. At this point I think your best bet would be to go to your local home store and search the plumbing section racks for an identical part. Better yet, take it to a plumbing supply store and they'll be able to match it. Good luck.