Thanks Bill, I have Balls book, fifth edition, and a few articles that I have gleaned from the internet, but am always suspicious of those.Done. Go see pretty pictures!
I bought the 2015 English language book Zbrojovka Brno - From Firearms to Tractors but was disappointed. It says very little about rifles.
The best English general reference is Mauser Military Rifles of the World, Fifth Edition, expecially John Wall's charts therein.
Are the 1937, 38,39 dated rifles all "stock" inventory for misc export orders. I know that many of the 1937 dated rifles went to China, and some to Japan. I "think" I have a dated receiver rifle
that has a Romanian prefix, but I may be "misremembering" as Hillary said.