What thread size these enlargers use on the lens board?

Origin of the M39 x 26 thread-per-inch 55º Whitworth thread Leica standard

https://en.wikipedia.org/wiki/M39_lens_mount

Nit picking a bit, but strictly speaking "UN" stands for "Unified" not "Unified National." (Want to acknowledge Petrochemist's post number 19.)

See ASME B1.1, a recent but not most current copy of this standard can be seen here: https://www.academia.edu/34623825/Unified_Inch_Screw_Threads_UN_and_UNR_Thread_Form_ASME_B1_1_2003

To see the current standard (2024) you have to buy it. Probably not much changed from 2003.

Rather than get carried away by my fascination with screw threads, I'll stop here except for this: Many years ago, shortly before he died, I emailed Steve Grimes a question or two about this subject. He replied back that he was always happy to discuss the "miracle of screw threads." "The miracle of screw threads" — my kinda guy.

David