Ian not sure.I will have to ask and iam sure people break these lathes and sell the bitsI doubt you will find any one who has one spare.
An interesting metal work exercise with piece of steel and files as might be given to an apprentice in the days of yor. What thread is it? (Diameter and pitch) Most likely bsw or bsf given its parentage.
You could try D Middleton stainless. He does lots of imperial stuff. Wing nuts were often used as adjusters for brakes on old motorcycles and so forth. Alternatively just get a metric one in a suitable size and drill out and recut the thread to suit.Iam looking for an origanal butterfly wing nut for the motor houseing bracket No 60 on the exploded diagram