Sorry to disagree with you Breplum, but if you zoom on the pic with the stop tube you can just see the white of a 1222 cartridge, coupled with the the crystal knob pointing down leads to a Posi-Temp valve.
OP if you can only turn the knob counterclockwise for on/off and temperature, not push/pull for on/off, than any Posi-Temp trim will fit.