spotshuntclub-
Apologies in advance if you've already learned the following:
There's a catalog reprint from the period of your rifle available from Cornell publications. (See image linked below.)
Supposedly they're still in busines at 570 Washington St, Wrentham, MA 02093 (508) 384-3791
It would be interesting to know whether the "Johnson" part of Johnson-Gartman was Eric Johnson, who at that time I think lived in Connecticut.
--Bob