Hi,
I tried: http://armscollectors.com/sn/marlinlookup.php with no success to determine age. Serial no is M121xx.
Any experts out there? It is not a golden trigger. It is factory drilled and tapped on left side of barrel right after receiver for what I understand Weaver or Lyman side mounts.
Thanks,
Smiley

This might help you narrow it down a little.

http://en.wikipedia.org/wiki/Marlin_Model_Golden_39A

if it's a 39A it was made in 1955