This is pretty close, I got the original SKP file in the Sketchup warehouse, I believe Johnny Round Boy did the original model.
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I have not checked all of the dimensions against the rail, but those that I have were very close. For example the slot on here is 8mm wide, my rail is about 8.25mm.
So to answer your question, the bolt diameter can be < 8mm or 5/16". The problem is the bolt heads are too large, 5/6" is 1/2" and 8mm is 13mm. Neither fits into the slot without being ground off slightly. A 1/4" bolt will fit in the slot, but it may spin, I have not tried it.
RMW