Re:
Thanks for input guys, did some measuring earlier and "test fitting" - one side fits if you go past the jaws, other side won't.
Wheel axle diameter - Left 8.93, Right 9.09 - so taking into account measurement in-accuracy 9mm - same a my more modern wheel sets. Also measured a very old (read early/mid sixties) nutted axle wheel - inc threads diameter was around 8mm.
Drop-outs
Left, where the axle should sit, 8.6mm, going down to 8.4 at the jaws, 8.0 including "burrs" on both sides of one edge.
Right, where the axle should sit, 8.6mm, going down to 8.5 at the jaws, no burrs.
Looking more closely at the axle end, definitely no threads but does look as though someone might already have taken a file to them.
Next steps will be to remove the burrs on the drop-outs and see where we are - might be a few days, got two other bikes needing more immediate attention.