Are you after the valve or the unit with the pistons in ?
The piston units on the later models have square seals on and are not available or replaceable as I believe they are factory moulded on .
The earlier model boxes have o rings on the pistons so you need to get the unit off by taking the selector fork lid off and undoing the 2 bolts in side then pull the lot out and swap it for older one .
I don’t know when they changed or how you can tell without looking inside but older ones have the speedo pick up on the angle and later magnetic drives point straight up .