Old Mixet shower valve

We have a pre-1968 Mixet valve in our tiled shower stall. Several years ago I replaced the o-ring and washer, and made notes of how-to for the future.

Now that it is not providing a usable temperature range, i.e. never hot, I was scoping out parts to replace the old original(?) stem. A diagram on a stem package made me wonder because it shows two keys along the length of the stem. My recollection and notes only indicate one key. As I looked at the new stem, I only saw one obvious place for a key (one slot) within its length.

Does anyone there know if it needs two keys? If so, where to get a 2nd one --and where it fits in? (I have doubts because the diagram also seemed to show the stem going into a side "two handle" position, not the center of the valve. Made no sense, and did not give me confidence to buy that stem.) Don't want to tear it apart without the needed parts in hand.

It sounds like you already know about the different Mixet parts. You need the MS3, not the MS5. One thing to look for, some hardware stores now may carry the MS5 and the MS-EXT which is a long shank version of the MS5, for tile or fibreglas installations, where the valve is back in the wall too far. It is long like the MS3, but is NOT a replacement for the MS3. The older MS3 is getting a little harder to find.

Should it just have one "key"?

Thanks, Jimbo. I didn't notice a stem number, only the fact it is for pre-1968.

I'm still puzzled about where a second key might fit in farther along the stem, or if that's a mistaken sketch. Single key or two keys needed?

--David