I recently ordered a speedometer cable for my 29 M
Sounds like you have one of the odd-ball speedometers. There was only a handfull of speedometers manufactured like what you have with the male threads coming out of the speedometer head. Those speedometers were manufactured by Northeast and I believe Waltham also manufactured one with male threads as well. They were such a low production part that no one is currently reproducing cables/housings for them. You may check your yellow pages in your phone book to see if anyone around you does speedometer cable repairs. They may be able to make a completely new cable for you or modify the end on what you purchased.