The nut which holds the pulley onto the armature shaft of a Lucas generator on my 1947 Ferguson is missing. The threads appear to be 21/32 inch, 20 to the inch, but they don’t correspond to any thread tables. Do you know what the correct nut is?
According to my parts books, the nut used on generators (Dynamos) prior to serial number 132206, which would include your TE-20 or TE-A20 is MF part number 829862M1 which subs to MF part number 1006935M1. This is the same nut used on the Delco Remy generators (Dynamos) used on the TO-20, TO-35, TO-35, F-40 and many other US built models. That same nut was also sold as MF part numbers 1750393M1 and 1501680M1. I am fairly sure the thread is not a common thread. I have a specification for the thread size but I do have several of these nuts on hand. I would think a local automotive electric repair shop or starter alternator repair shop would have one of these nuts on hand as well. If you cannot locate one I can send you one, though I am not sure what the postage would cost.
The later TE-A20 generator (Dynamo) uses a 7/16″ x 20 nut, original MF part number 829809M1 which subs to MF part number 353428X1. If your original generator (Dynamo) was replaced with a later one it may use this other nut.