While not directly related to the OP's question, I found something a little intriguing recently in the photo collection of the Springfield Armory National Historic Site. There is a Springfield Armory photo of a M14 NM with a ten round magazine. The photo shows the left and right side views with a 10 round magazine inserted into the rifle in both views. The caption is dated 6 April 1962. The TRW M14 NM rifles were manufactured after September 1962. So, the photo is of a Springfield Armory M14 NM rifle. The photo caption reads in part, "RIFLE, 7.62mm, M14 with NATIONAL MATCH APERATURE and 10 ROUND MAGAZINE."
So, my question is, was this 10 round magazine shown in the photograph manufactured at Springfield Armory? AFAIK, this is the earliest dated documentation of a M14 / M14 NM 10 round magazine.