I apologize....looking at the other markings on your rifle i have misquoted the production date I'm afraid.
the markings on the receiver are:
3,67g GBP meaning 3,67 grains of military flake powder.
ST. m. G. meaning steel jacketed bullet.
this style of markings were used from July...
sorry to find your post so late but just got on site...hope you're still around
15978 is the serial number of your rifle. many small parts will only have the last 2 numbers of the serial number stamped in them....but the Germans loved to number everything and if perfect they will all match...