They've arrived:
They're a bit smaller than I expected but very workable. The detents count out to 10 stop positions within one turn; about 300 degrees. I mounted one through a piece of sketch board to see how they work. There's a tab that holds them in place so they don't spin on their own. Mechanically the feel is very much like that of a rotary switch which is what I wanted. Very smooth.
I don't seem to have any knobs that will fit the D-type shaft so I'll need to order some of those. And I need to test them out with an Arduino to find out if they will produce consistent values to get the 1 in 10 selection result. That comes next.
