Thumbscrew for tonearm weight. I have a Scout 1.1 and I need to know the size of the setscrew so I can buy a thumbscrew (see picture)

We always used a set screw so you could really wrench down on locking in the counter weight. However, it wasn't as easy for adjustability which led to eventually using the thumb screw. Regardless both approaches work and will lock in your counter weight to keep your tracking force accurate.

why do want to change the set screw for a thumb screw? Personally, i prefer the set screw