I have seat track springs, rollers, etc... but I've been down this road before. Long ago I answered a request for that same spring. That member was in Australia or Austria, I forget. When we found out what shipping would cost, it didn't fly.
Any local machine shop can turn a standard 1/2" bolt into that special shoulder bolt. The right length of bolt will have close to the right length of clear shank. The 3/4 hex head is standard.
That spacer is made of steel and rubber like a motor support. It puts a little tension on the pivot so the seat back doesn't move as easy. At one time Year One had those. I added one only on the outboard side of each 67 bucket. Works just fine.
