According to Rule, the G prefix was added as a result of a government requirement.
I don't believe that's true. (For example, the Model 94 didn't acquire an alphanumeric serial number during the same period, nor did the 101 shotgun, though the Models 88 and 100 did.) The only "government requirement" for firearm serial numbers was the '68 GCA requiring serial numbers for firearms retailed here in the U.S. (Some manufacturers previously didn't have serial numbers.) Rule also states that this didn't begin until August 25, 1971 at serial number 1,028,977 (page 319) and it's fairly apparent that that's not true either.
From page 570 of Frank de Haas'/Wayne van Zwoll's "Bolt Action Rifles, 4th Edition:"
"Model 70 rifles with a serial number above 866,000, and with the letter
G preceding the serial number, are the 1968 version with the anti-bind feature, black-chrome floorplate and stainless steel follower."
