I had the lock for my ''47 3100 but not the connecting shaft. Got some 1/4" square stock from the local hardware store. Cut it and filed it to size and flattened one side and it worked fine. Ever was able to have the locksmith make me a key since I didn't have one at all.
Check to see if the mechanism is gimmes up with old grease etc. perhaps squirt something like brake cleaner on it to make sure it's "clean". Then........f
I guess I would first try a lube of sort-graphite, oil, grease on the actual
Lock mechanism. It sounds that the handle of the screw driver may be giving you more torque than by using just the key. The cleaning along with the lube may just do the trick. Try with the screw driver, then with the key. Hopefully it will then work!
Also be sure that the shaft is correctly sized.
Good luck.