Grub/lock setscrew size, Solidoodle 2 pulleys?

Anyone know the correct size of the locking threaded setscrew hex-socket jobbies for the aluminum collars that hold the pulleys in place, on the Solidoodle v2 (ancient original version with the Sang board if it matters)?  Lost one at one end of my belt loop controlling X.  Machine still works but I can see the belt end and pulley 'rising' under tension as the carriage heads toward X=0 (right side), where it sits on the screw in the right side Y axis carriage, and that's probably influencing my movement accuracy some.

Thanks in advance....since I can't seem to find small enough parts local I'll have to order and don't want to just guess on a few sizes.  (There is a really great mom-n-pop hardware store near me called Nuts and Bolts that does have all sorts of great metric stuff, but the smallest setscrew they have is like 4mm which looks far too big....)