Just my experiences from the period and it was the stock Mattel supposedly made.Do you have an "authoritative source" that any part of M16 WAS made by Mattell?
FYI my BPED was 29 Oct 1956, DOR as CSM/E9 26 Oct 1982.
We weren't a line unit so we got 16s without the forward assist. Fortunately about 3 months later we got the A1s. During my career my TO&E weapon was a Garand, carbine, M14, M16 and .45ACP. They were just bringing in the M9 Beretta when I retired and I was still carrying a 1911 in a shoulder rig at the end. Yes, I've got a 1941 WWII vintage belt holster I was issued somewhere but avoided using it as it was right handed.