BobT is a collector that frequently lurks on the forum, keeping us honest, much like Jon Speed, he emailed me an hour ago when he noticed similar batch codes on the bolts... I had overlooked this until he brought it to my attention, but going through my database, guided by the trend sheets that referred to matching bolts, I came across a solid match (8D7) to link these batch codes on the receivers to the bolts. Here is a Amberg/17, I think it might be Wolfgang's, but not sure.
Amberg 1916 8339 b
Unders is unknown letter for me. Probably S or R ?
Thought I would contribute. Code under receiver is '8G7 - O' Code under barrel is 'G'
Hopefully the pictures are good enough to see what you need to know.
Not sure if you are still tracking these, but here is my 1917 Amberg: