The vast majority of “dual codes” Mauser-Berlin assembled were based upon Astrawerke made receivers coded “bcd”, which represents Gustloff-Werke’s operation at Weimar. They range in dates from 1941 through 1943, though all seem to have been made in 1942 and 1943. These receivers are far more complicated to date than the ERMA “dual codes”, which all fall in a narrow window of production. Apparently Mauser-Berlin took delivery of the first receivers very early in 1942, you see them showing up in the middle of the first block of 1942 production and increase in number through the early blocks of Mauser-Berlin production. Actually, this type of receiver represents the majority of rifles assembled in some blocks. It is far easier to find a bcd/ar 42 in the “b” through “g” blocks than a straight up ar/42 in those blocks. These seem to end in the 1942 g-block, at least for a time, only to show up again in smaller numbers in 1943. During 1943 the bcd/ar 43 dual codes are much rarer, but show up in almost every block between the a-block and j-block (they show up in the b-block through g-block, at least one is known in each block, plus one i-block is also known). Most interesting are the receivers that are not actually “dual” coded, there are several Mauser-Berlin assembled bcd/43 without the added “ar” in the last blocks, the g-block and i-block, which shows that they dispensed with this added feature near the end. The bcd/ar 42’s and bcd/ar 43’s are trickier to date, obviously, the 1943 dated were assembled in 1943, the barrel codes on enough of them prove this, but as the acceptance and serial patterns are nearly identical between 1942 and 1943 production, you really need to see the barrel code to know when a rifle was probably made.
A collector interested in this variation should always keep an eye out for g-block through n-block bcd/43’s. It is entirely possible other bcd/43’s could exist made by Mauser-Berlin in those blocks, yet lack the added “ar” in the manufacturer’s code. It is always a good idea, on later production, regardless of maker, to keep an eye out for the final acceptance. It determines who made the rifle in all cases, the barrel codes and serialing patterns can also help in many cases.
As Mark Wieringa wrote in 1994, these rifles were not a joint venture between the two firms, but rather Mauser-Berlin using subcontracted receivers, which were simply intermingled within certain ranges of their normal production. Probably based upon availability, it was not a cooperative effort and little different than other such production methods used later in the war (the use of other makers receivers due to production or transportation difficulties). Anyone interested in this subject would benefit from reading Mark Wieringa’s article, in the April 1994 KCN, his discussion largely revolves around the marking pattern, which I have not covered here, but his outline does a good job explaining it. I have not bothered with classifying these features in my study, but it might be worthwhile to consider this feature in future researchers work.