Dynascope rotating ring tube tightening screws
Here's a few pictures of one of the screws and nuts that hold the rotating tube assembly in place on the Criterion Dynascope 8 inch Custom Deluxe telescope. The hex screw is 16 TPI x 5/8" dia. x 7/16" long. It uses a 3/16 inch Allen wrench.
These screws are used to adjust the rotational friction of the tube. Do not over tighten. Conversely, I get the impression the tube will slip out, risking serious damage, if enough screws are loosened too much. Something to consider if your going to work on the rings.
The hex nut is an ordinary type that serves as a locknut for the hex screw. It looks oversized in these pictures but in context with the entire scope they look OK.
The screw presses down on a white polyethylene like plastic material visible below. The original screws have cupped tips. A smooth flat tipped screw should work fine.