Two K98k rifle manufacturers provided receivers for another manufacturer to assemble into a complete rifle.  They were Gustloff (bcd) and  Erma (ax).  By far, Gustloff provided the most.  Erma sent receivers to Mauser Borsigwalde (ar)  and Gustloff sent receivers to Mauser Borsigwalde but also sent a few to Steyr in Austria  (bnz).  The receiver maker's code was already stamped on the top of the receiver so the assembler added their code.  

The known "dual codes" are bcd/ar, ax/ar and bcd/bnz.  By far, the rarest of these is bcd/bnz.  Very few have been reported.  The assembly by Steyr is verified by the bnz added to the top of the receiver and  WaA623 waffenamt stamps on the bolt, receiver, barrel and rear sight.

This example was duffle bag cut.  The cut was made below the barrel band in plain view.  This may turn a lot of collectors off but, the way I see it, this rifle would not have made it to the US had the GI not cut the stock.  It would have been nice if he had figured out that cutting underneath the barrel band would hide the cut.  The cut was very nicely repaired.

Picture Set 1

Picture Set 2