Wouldn't it have to be 1972 or after if it was a Security Six? A Gunny attached to the Marine Cadre that oversaw ASF commented on the Security Sixes carried by base security (this was 1985). He said the Marines issued them for embassy guards and MPs. I was shocked. That was early in my career and I thought everything would be 1911. Man did I see a ton of stuff being carried aside from the 1911 up until the early 90's.
My shooting buddy, and family cousin is retired USAF Colonel, B-52 pilot and later Nuclear Missileer. in the 70's and 80s. When I asked him about issued sidearms, it was a hodgepodge of 1911's, S&W Model 10, and later Beretta M9.
In the nuclear silos, on rotation duty, it was" whatever they had available in the arsenal".
So, 4 days on/ 2 off, whatever sidearm in the rack was issued. I find that odd, that sidearms were not standardized for training, but then, Minuteman III nukes were probably more the focus.
He preferred the S&W Model 10, and has one now, that was an early "custom Shop" revolver, coated in Birdsong Black -T and tuned action that is a maiden's kiss.