I think the 'G' was added in 1968 when the mods were made, anti bing slot, steel floorplate, and some others.
source is Frank DeHaas book onBolt action rifles
from The Rifleman's Rifle, by Roger Rule, pg 319:
"On August 25, 1971, at serial number 1,028,977, the serial number sequence was given a "G" prefix. This was implemented in compliance with new Federal regulations requiring a letter-coded serial number as an expedient means to manufacturer identification"
(Guess the Feds couldn't tell a M70 form a M700.
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