I am no expert on serialization of Winchester Model 70's. So I went to one of my books (The Rifleman's Rifle) to see if I could find the answer, and to educate myself in the process. I looked at all the references to serial numbers that I could find. Production of the Pre-'64 Model 70 began in 1936 beginning with serial number 1 and continued through 1963 with a final serial number of 581471.The Model 70 was redesigned in 1964 and there was a jump in serial numbers to begin with the serial number 700,000.

When I looked at the serialization charts in the Blue Book Of Gun Values I see the "G" appearing on serial numbers beginning in 1969. But, those serial numbers began at 941,900-1,537,134 in 1981 so this doesn't explain your serial number. So I went back to The Rifleman's Rifle and began reading more.

According to the above reference, " After August 25, 1971, any 'Pre-64' receivers sold from parts inventory were required by Federal law to have the manufacturer's assigned letter identification code prefixed to the serial number. For Winchester, this was a 'G'."

It would appear then from the serialization tables, that the receiver was from a parts inventorymanufactured in 1939 and bought sometime after the August, 1971 date from above. I am sure there are many more people on this forum more knowledgeable that can expand upon this or correct me. I hope this helps a little.