Found a few tidbits
here that might help:
* The takedown screw head didn't protrude after 1937.
* After production of @ 125,000, the triggers were grooved.
* After production of 134,000 the tirggers were ungrooved.
* Early guns had bolts, triggers, safeties, cocking pieces and trigger guards chromed.
* Earlier guns had finger grooves on forends.