Camming pieces (trunion is a misnomer) are numbered to themselves only, not to the gun.
Here are the manf codes and dates, someone (Steve, Tom?) should make a sticky out of this info.
AR= Mauser, J.T. is 1943, D.F. is 1944, D.F. with dd mark is 1945
dfb= Gustloff, F.G. is late 1943, M.U. is 1944, S.M. with svq mark is 1945
cra= Maget, GH is 1943, NC is 1944, NC with swd mark is 1945
bnz= Steyr, GZ is late 1943, PJ is 1944, PJ with swj mark is late 1944, XE with swj mark is 1945
bpr= Grossfuss, only used in 1942
As noted in Myrvang's book, pg 134, the marks indicate the factory that assembled the guns as Grossfuss stamped most of the receivers for all but Steyr.
this info provided by dan@angola armory