Thank you ahead of time for helping me with my predicament. I believe the previous owners did a 'do it yourself but badly' attempt on the shower with this knob, as I have been unable to find any images or pictures online of a on/off knob like this one. I am not sure what this kind of knob is even used for traditionally. In any case it has a two part turning mechanism. The first part turns the water on or off by screwing it to the left. The second dial behind the on/off is temperature control. The entire device is plastic. I've replaced it with one I found at Home Depot that they had in an old storage box for valves but now I really want to put a regular nicer looking on/off - temperature - knob inside the shower.

I took a picture and added it to the post for reference. When I bought a new kit for the shower, the problem was the fitting did not screw into the pipe. It has been awhile since the last time I tackled this project but I believe the male thread on the new kit was bigger then female threading on the main pipe running up through the wall.

Any experience with this type of change?

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Hi BullMouth: It is Mixet. These faucets used to be widely used in Commercial / Apartment applications, not too much in residential. They are still in use...

Trim used to come in 2 sizes: the narrow one you have and regular one, about 8" in diameter. All parts, including the knob, cartridge and trim can be purchased in Home Depot. Knobs, including the lever, can be "crystal" plastic handle or Metal.