Anyone recognize this Amer. Std shower cartridge?

I have and old rental house and need to replace this old (20+yrs?) American Standard shower valve cartridge in the photos. To make this job more of a pain for whatever reason the last owner did not install supply line shutoff valves, so have to turn off at hot water tank. Am hoping to ID the cartridge before I start pulling it all apart, as am concerned it's so old it may be discontinued, so any help is appreciated. Also, do i need to unscrew that chrome ring before i try to remove the cartridge retaining nut?