Model 90 with "G" serial number was made in 1950. Marlin started using the letter prefix in the serial number to indicate the year made after WWII. C-1946, D-1947, F-1949, G-1950, H-1951, (Marlin did not use I, O or Q) J-1952, etc. Apparently, there were a few made prior to WWII that were marked with A (1940) and B (1941)
In case anyone is interested there are a couple of Model 90's listed on Gunbroker.com, a 16 ga. Sears Ranger 103. and a 12 ga. Model 90ST. Search for Marlin 90 rather than Marlin Model 90.