This is a picture of the brass washer before I sanded it to the tolerance I needed. And you can see the shim in the other. Just a copy of the U shaped locking washer in the front with the three screws in it. Made of brass. You have the main screw with the handle. That has the shoulder that sits against the casting in the front jaw. Then the groove behind the shoulder on the screw that the locking U shaped key rides in. This appeared to have a fair amount of slop from new as it was not worn all that bad and I just wanted to tighten it up. Putting the brass washer between the front jaw casting and the shoulder on the screw was perfect leaving about a 64th of tolerance from the backside of the shoulder and the front of the jaw. But then I had a larger gap which caused an 1/8th in gap when reversing and I was not happy with that. Hence the shim behind the locking plate that took that up. Overall now, virtually zero play in the jaw and being brass it runs super smooth. Also, just the brass edge of that shim showing is a detail I like. Most will never notice but anyone who does this stuff will and will understand how much work actually went into this thing.
I thought the shim was going to be a lot tougher than it was. Once I discovered rummaging through my hoard of junk that I had something to make it from. I roughed it out then sanded it with a die grinder and had it done in no time.