Daz.....the real issue here is understanding what is what........
The sample hollow pointed grub screw may measure
1.5mm diameter however the ISO Standard 4026 tables as below confirms the actual thread size for this item as M1.6x 0.35P with an in hex of 0.7240 AF
Manufacturing tolerances at these relatively small sizes makes measurement difficult to understand especially the in-hex size
It appears
metric grub screws are manufactured & available as M1.0, M1.2, M1.4, M1.6, M1.7 & M2.0 ...and also in slotted head
....again, M1.5 diameter grub screws are not listed
I have used a dozen or so M2 x 0.4 HPGS.......the manufacturing tolerance allows for a minimum thread size of 1.87mm diameter and the in-hex is listed as 0.9020, however my M1 driver fits the HPGS just fine so you can see where the measurements confuse the matter
Derek