Considerably late on this Thread. But worth an answer. The "G" series prefix was part of a new serialization scheme as Winchester adapted to mandates of the Gun Control Act of 1968. Now, "G", integral in the serial number indicated specifically the Model 70. Other Winchester models had their own differing alfa prefixes.
From that point forward, Winchester produced such serialized rifles, Yet for reasons unclear, the firm set aside some entire serial number blocks. Thus 'holes' in the serial production. No one to lose sleep over it.
In the early eighties Olin Corporation, Winchester's parent firm, licensed production to U.S. Repeating Arms who undertook future Winchester production and initiated new models in the process. Eventually, USRA decided to utilize those serial blocks formerly reserved. Thus the apparent out of sequence numbering created. Such both confusing and erroneous when trying to apply sequential/chronological logic to the result. Then add the final point. USRA has never released their serialization-production date information.
Nowadays, the best evidence of USRA model production data generalizations are such as catalog information and individual new purchase recounts.
The bottom line here, if your Winchester serial number to published data a 'tilt', recall that if it's a USRA gun, you're in good company. And now you know the rationale if not the answer!
My take
Other than the parts about the "G", and USRA taking over production in the early 80's, this is incorrect.
I can tell you within a year pretty much exactly when ANY Model 70 was made in New Haven CT.
There were no set aside numbers. They ran from the VERY beginning (1936), add "G" in 1968, through the VERY end (2006). EXCEPT 1991-2002 Classics.
Because "G" was NOT USED until 1968, the Classic series started at G1 in 1991, through about G380000 in 2002. These are the most sought after Classics.
In 2003, among many changes that year (mostly bad) the Classic series suddenly merged into the push feed (original) 7 digit run and remained there until the end.