There should be a set screw in the door frame pointing at the lock cylinder. Hope this helps. Normal Norman
I think Ford used the set screws up through 51 at least as I remember them from taking the locks out of my 51 Merc. Here is a thread off Ford Barn on the subject. http://www.fordbarn.com/forum/showthread.php?t=20473 There should be a hole in the door frame that is even with the lock that you can slip a screwdriver or allen wrench in if memory serves right.
Small access hole inside near door edge.Careful,that set screw is a slotted head(if orig) and if frozen you risk breaking 1/2 the slot off.Then its a real pain to drill it out'.
For reference only,my 48 had a spring clip that slide in a groove on the lock cyl.Access was thru the inside door panel. 39`s may be different Good luck
My '41Ford has a set screw in the edge of the door, After getting it out I replaced it with an allen set with anti-seize.