Hi John,The nut on my machine is 1 1/2" outside diameter and 1/2" thick, with the outside knurled. There is also a 1/4" set screw with a brass pad on one side. It's threaded 3/4" UNC, so looks to be slightly different to yours. The spring is on the left inside the dog clutch on mine with the nut bearing up against the right hand side of the handle. I've included a photo.
That serial puts yours as mid 1973. mines a late 77.
All the best
Graham
