From 'Bolt Action Rifles, 3rd edition, by Frank DeHaas, page 482
"Winchester rifles with a number above 866,000, and the letter G preceeding the s/n, are the 1968 version with the anti-bind feature, black chrome floorplate and stainless steel follower."

Also from, www.wisnersinc.com/additional_info/Winchester_64_M70.htm,
"After the1968 Federal gun control law----, the post 64 guns then carried aG s/n prefix starting near 874,000.