* Beats me! First week of May, 1942, give or take a couple of weeks.
* Anything more precise will take far more documented knowledge than I presently know.
* Jan Still, in TRL
-Pg 66, gives byf 42 production as 113,000 for German Military procurement & another approximately 10,000 for the Portugal/Bulgaria contracts. The Military S/N range given is 500_ to 149m, 149m-5322m for the Portuguese, and 5322m-2197n for the Bulgarian Contract.
* Sloppily ignoring a bunch of suppositions/facts and assuming constant linear daily production over 24/7 to simplify the math suggests about one letter block per month. Hence the answer above. Isn't new math wonderful!!
* The supposition/facts add intrigue to the definitive answer you desire. For instance, byf'41 receivers were still being assembled in the no suffix(ns), "a" block, and slightly into the "b" block and overlap byf'42 examples. Also, although not achieved, Mauser was targeted to cease P.08 production about June, 1942 when P.38 production was to replace the Lugers as the standard Army 9mm. This suggests a ramp down/phase-out of P.08 production and a ramp up of P.38 assembly. However, the first 700 delivered P.38's from Mauser aren't shown until November, 1942 causing Mauser, its logically thought, to continue P.08 assembly at least through Nov., '42 for the Army. Stategically, the Russian front was ongoing with attendent pistol losses rising. Tactically, material/transportation shortages, winter weather shutdown/slowdowns, the ever present pistol repair population, and the ebb/flow of detail manufacturing feeding Assembly can account for enough variation to con-boggle-fuse the math.
* If only byf '42-#135x d-9mmP-P.08 could talk! Pretty good looking old magpie she'd make. Thanks for sharing.
Good Luck & Great Hunting! Bob