Food for thought: (1) Why not asking Heym directly, they are still trading, they have a well constructed website, and I am sure you would get the correct information directly from the horse's mouth? (2) Your rifle is marked 'Made in Germany', so it may be even be younger that you think as if built between after WW2 and 1990 (BDR & DDR reunification) it should either have been marked 'Made in Western Germany' or 'Made in the DDR' (note sure about that one) ?
Louis