Thank you chaps!
Peak4 – A/F and Whit. spanners don't fit them! They take 17mm spanners even though not M10 threads.
NDIT – I thought of that but unfortunately found only I have no sample UNC nuts. Nor could I examine the thread forms fully enough. Howeve the 1/2" – 13 you quote is UNC, differing from BSW (12tpi) and BSF (16tpi). I keep a Tracy Tools poster-size chart by my computer for good reasons!
Martin – I did try thread gauges but could not obtain a convincing match with any of them.
So, overall, it does look as if UNC is the more likely, and the easier to obtain.
Very often, using a set-screw or bolt is better than the stud, especially where the height is restricted.
'
Spanner sizes, I am afraid, are no guide to sizing flanged nuts, both on these clamp sets and those bought as general fasteners. The nuts on my clamp sets take a 17mm A/F spanner, standard for ISO-M10 Coarse, but the nuts are certainly not M10. (An M10 bolt will not even enter the thread.)
My club's portable track fishplates have M10 or M12 flanged nuts readily available from builders' stockists; but the forged, heavily-tapered hexagons are not those established for their standard M-n Coarse thread, Fortunately the spanner they accept is readily available, just not the normal one. I found this too with some of the building-trade fastening holding the overhead-hoist columns to my workshops' concrete walls, and I think their nearest spanner was a Whitworth despite the fasteners being otherwise metric!