I have a nice Helios vernier caliper that lacks only the thumbscrew that
clamps the precision adjuster.

The fine adjusted is the right-hand clamp screw that sticks up in the
upper diagram in
http://www.gagesgalore.com/Fowler/Calipers_Vernier_Selections.htm.

The screwthread appears to be M2.5-0.35mm.

Anyway, anybody have any idea where to get a replacement thumbscrew?

Thanks,

Joe Gwinn