The confusion might come from the fact that this thread is often written as M39, when it really is 39x26 TPI, no M. It's a Whitworth thread, with a 55 degree profile angle, only the outside Diameter of the male thread is given in mm.

I have a few lensboards made to this specification (26 TPI and not 1 thread/1 mm) and I can screw in my factory LTM lenses with the greatest ease.