Trailers are made from all sorts of sources, even today -- I'd swear some of the trailers I'm seeing now look like they were made from HD Rec.709 dailies.
Back in the pre-D.I. days, if the movie had been completed, the marketing department probably got their own dupe negative made from the final color-timed interpositive that was struck from the o-neg. Then they would cut a trailer from a work print struck from the dupe negative, then cut the dupe negative, and then have to make an IP and then multiple dupe negatives from that IP because of all the trailers that would have to be struck.
Even if the movie was still in post, a trailer department wouldn't be allowed to touch the original negative other than to order an interpositive made of selected camera rolls (remember, the o-neg hasn't been cut yet) or a CRI (color reversal intermediate, thus a negative made from a negative) back in the 1970's. And considering the hundreds, if not thousands, of copies that had to be made for the theaters, they'd have to print those from dupes of the original dupes.
I remember back when I did "Akeelah and the Bee", I found out that the marketing department had ordered their own IP struck from the negative as soon as negative cutting was done, a one-light that they later timed themselves while I was doing the answer printing of the movie independently of them.
But not being a trailer maker myself, I am only guessing as to the exact workflow.