What size countersunk screws for Audio Research Ref3

Maybe contact Audio Research

Measure the diameter of the threads on the hole. They will fall into a range of the mating fit thread. Just do a quick google search and you will find it.

Screws are standards, ARC will not spec a special thread or diameter.

For example if the hole measures 0.189" then it is a 1/4in thread after that just need to determine if fine or coarse pitch

My point was a quick phone call to ARC would probably get his answer.

I have all arc. A google search says the screws are 6-32.

I would contact ARC.   See if they can send you some.  Countersunk screws have either 80° or 100° bevel.  A 100° bevel in a 80° will stick out and a 80° will not be flush in a 100° bevel.   Best to get the O.E.M. screws