I think it was an "official" rating as I believe MPAA assigned it to pics like Midnight Cowboy and the original release of A Clockwork Orange, but MPAA never trademarked it which led to it being co-opted by the adult film industry and the switch to the trademarked rating, NC-17.
In historical...
Every box office info site seems to have different numbers for films (even current titles). I'm not sure any of them can be relied upon for absolute accurate numbers, high or low. Especially worldwide totals from 40 years ago.