[Ford-ferguson] Ford 601 Workmaster ignition key

[cwi813 at earthlink.net]

I'm really new to antique tractors. I have a 1941Ford-Ferguson 9N that I am enchanted with, and I've been saving all your ideas and comments in case I need them for my tractor! Thanks for all the help.

I'm trying to find an ignition key for a Ford 601 Workmaster tractor. I don't know when it was built (maybe 1952?), but someone stole the key from the ignition when it was accidentally left there overnight (the front hood emblem was also ripped off).

I've had two locksmiths say they "had the right key," but they don't work. 

Could you tell me where else to start?

The information I've gotten from the Workmaster is below. Directions "left" and "right" are given as if you were driving the tractor:
Right side, farther down under the gear shift on the flat side of the casing/housing side. It says, "DIF 310835." Below that it says, "3" then (and the following numbers are cast in the metal upside-down)  0028:           
Again, if you were sitting in the tractor seat, the following numbers are under the seat area, on the metal vertical housing/casing on the left side:  "DIF  312478"     
I would be delighted to know what year it is, and how to find a key that works. I don't think the ignition has been changed, but if I can't find a key, where can I go to find a new one that will fit?

Thanks for all the expert help I've read so far, and thanks again for considering my problem.

Sincerely,                                                            
C.J. Wilhelmi
cwi813 at earthlink.net
515-278-5937
515-707-4082 cell