Yeah, mosh pits really werent a late 60's/early 70's "counterculture" sort of experience a rock fan attending festivals or shows saw back then, also if you examine how rock crowds acted or behaved during that same time period compared to later on in the late 70's onwards, they tended to be more restraining, less unruly even if security regulations at early rock festivals like Monterey, Woodstock, Altamont, and Atlanta Pop Festivals weren't as stringent, or safety-conscious and disgustingly corporate they've become. I think when most major rock bands began playing stadiums in the early-to-mid 70's, and harder drugs hitting the mainstream like herion, cocaine, PCP, methaphetimes mixed in with 50-60k fans, that's when crowd control became a real, existential issue regarding entertainment or rock acts.
As far as "mosh pits", thats mostly a result of mid-late 70's UK and US punk bands' fans that liked to ram and collide into another to sort of mimic the violent, fast-paced, hard sonic sound of punk and then later on, hardcore punk bands like D.C.'s Black Flag, Fear, Minneapolis/St. Paul's Husker Du, The Minutemen, Agent Orange, and last but certainly not the least, Dead Kennedys. A lot of the New Wave British Metal bands and early 80's thrash metal bands kind of adopted and assimilated most of these norms. I know Metallica, early on especially when Dave Mustaine was still there had a very fast, punk-related sound and all of the original members of the band were huge fans of 70's punk bands in and as much as other thrash metal acts like Slayer, Anthrax, and Venom.
I'm curious, SF, considering D.C. had a pretty lively, competitive hardcore punk scene in the early-to-mid 80's, did you happen to catch some of Black Flag's early shows or with Henry Rollins?