Well......... best guess (tricky even under microscope) is 0.16" diameter by 36t.p.i. So possibly 1/8" by 36? Trouble is, there are only 2 or 3 threads visible, so difficult to be 100% sure. Checked with the tables, and none of the standards (Whit., UNC/F or BA) corresponds, as 1/8" is 40 tpi in Whit /UNC, or the nearest B.A. is 43 tpi! Nothing remotely similar in metric, so probably the usual "Stanley special". But with the short length involved, you could probably get away with Whit or UNC?