I know of only two versions of the glides, one is a white nylonish material with a slot on one end and a tapered flat point on the other. It was orginial with my 1976 from the factory. This in adjusted from the top of the of the compass head and extends as needed to remove the rocking of the head when sitting on the main table.(when replacing these nylon parts I found it best to start it backwards from the bottom and after the treads engage use the screwdriver from the top to position it.) A second metal set screw with the 5/32" allen sits above that. The purpose of the second set screw is to hold the rod in the head. It is normally kept atop the nylon looking one so it stays in place and will be there when needed.
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The while nylon bit is about 3/8" long but could have been replaced with a longer version but it can not be longer then 3/4" or it will interfere with the rod sliding in the hole. The metal setscrew is 1/4", again it could be longer. With it tighten atop the nylon one the rod will slip in without hitting, well providing the two don't add up to more then 3/4".
The newer version has a hex head with a slot. It is inserted and screwed in from the bottom. On the couple I have the screw is a loose fit and will not stay adjusted unless the metal set screw is screwed in and tighten. Now it could be it just the ones I have but if yours is like mine it helps to make sure the set screw is in place.
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In both cases the threaded portion starts about 1/2" down from the top with the area above being clearance.
I can not speak of early shopmith versions but from the 70's on, well that stuff I know.
Ed
). Well, you get my drift.