I have a 7mm Rem Mag in Win. m70 Custom Super Grade, controlled round feed, SN G1123. According to my shooting book it was first shot on Dec.20th, 1992. Based on this it was purchased just 2 or 3 days before since I always get them to the range ASAP.
How much earlier than this it was built I can't say, but based on the SN it couldn't have been too much before.
Maybe this info will help some of you.
Jim
