Looking at the left side of the hole, and assuming that is toward the back of the valve, it looks to me as if there is like a shelf in the bottom of the hole on the left side of the hole. There seems to be some dirt on it with some of it wiped off.
I don't recall seeing anything like a shelf in a Clack WS-1 but it may be your Hlllenbrand valve is a bit different than a stock Clack WS-1. If that were the case a stock stack may not have been used and you need the stack and pistons from Hillenbrand. I can't see the ends of the spacers on the left side.
Are you getting the stack etc. from Hillenbrand and if not then who?