I agree, a nice rifle, and not too common with original parts, many are rc’s.
As stated this is a build, not a rework, built at a depot in Wehrkreise XXI (Posen was the HQ for this district). The district being in the east was one of the more important districts once the Russian campaign started, as were the General Government (unincorporated Poland- Krakow, Warsaw & Radom - all critical operations with small arms)
Your rifle is very neat in that it has an ERMA ordnance spare barrel, the circle EE wasn't used long, but can be found on all sorts of ERMA manufactured ordnance spares. The stock is ordnance spare too; the receiver is Astrawerke, and you can tell it was never used before as it doesn't have RR acceptance (all Gustloff Weimar bcd/41 should, though late they moved the final to the top) or a top final as 1942 on will.
These builds were part of the effort to fill in for the rifle shortfall, caused by inadequacies of nazi economic policy. The issue went far beyond rifles, which wasn’t even an important issue as it was a military wide shortage, just utter chaos affecting every aspect of manufacturing. It took nearly two years to resolve the most pressing issues, and even still productivity was only fraction what it could have been.
These Astrawerke receivers I suspect were diverted to fill in the gap, to allow the depots, primarily in the eastern Wehrkreis, to supply rifles to the eastern armies. By utilizing ordnance spares & salvaged parts. Most known are from the eastern districts. All the eastern Wehrkreis and the General Government were key supply hubs for the German Armies operating in Russia, uniforms to small arms repair. Speer goes into this in detail, really a lot of activity and heavy ss involvement (due to the labor situation).
Generally ordnance spare receivers are blankos, most well known, Walther and BSW made, but during this critical time you see diverted Astrawerke bcd/41 & bcd/42 receivers, and some BSW/1939 receivers, both lacking factory applied acceptance. You also see a great many salvaged receivers, vz24 new receivers, old Gew.98 and even an occasional recycled early 98k receivers rebuilt with ordnance spares and marked to the Wehrkreis they were built in.
This bcd/41 receiver was part of that program; simply a diverted receiver originally intended to supply Gustloff Werke Weimar, who Astrawerke was contracted to supply in 1938. Had Astrawerke been contracted to supply the ordnance system directly with receivers, I suspect they would have been blankos, like the BSW or Walther receivers. As when Astrawerke directly supplied receivers to other firms, they marked them to the destination, - MB, BLM, etc.. when using Astrawerke receivers they are marked appropriately.