when purchasing the HP cartridge assembly without the housing you may require the retro kit for the new cartridge to fit in to your old body. This usually depends on the year of your shower or the old cartridge.
The retro kit will be needed in the case below.
If your old HP cartridge has two circular orings and two d shaped orings on the back of it then you will need to purchase the retro kit also.
There are also two different kits, One for the recessed model and one for the surface mounted model.
Please also check that if you old cartridge has no locating guide pins on the back of it please remove the ones on your new cartridge otherwise this will stop the new cartridge seating to the body properly.