I think 39cm was fairly common for non-suspension adjusted frames, although some were built for 38cm forks.
The initial suspension-adjusted frames were made generally for 41cm, more rarely 42. A 60mm fork will tend to have an unsagged a-c of 43cm, sagged c41.5, so building the frame for 41 was a reasonable compromise.
It's complicated by the myth that a 71 degree head angle is 'right', whereas in fact many earlier mtbs with 39cm forks had 70 or 70.5 angles. The head angle reduces by c0.6 degree for every cm longer a-c, so e.g. a frame built to give a 70 degree head angle with a 39cm fork would need a 37.5cm fork to give a 71 degree angle.
Also many makers cottoned on to the market belief that 71 degrees was 'right' and just said all their frames had 71 degree geometry, even if they didn't. It's called marketing. And back in those innocent times, many buyers would buy a 71 degree frame and then fit a long fork to it, thinking the head angle would still be 71 degrees. Which is called boll*xx.
One decent approximation of what type of fork a frame was designed for is to fit any old fork and a pair of wheels and measure the bb drop with that fork (i.e., the height of the bb centre relative to the hub centre). XC frames were invariably designed for a 30-35mm bb drop, so if you have a bb drop of say 15cm that is a clear indication that the fork is longer than the frame was designed for. The bb is about 40% of the way from the rear hub to the fork, so 15-20mm too high a bb indicates that that particular fork is 40-50mm too long, which gives you a general idea of what length fork to fit to give the intended head angle.
It also suggests that the head angle if you stuck with that fork would be quite slack - not the end of the world of course, just fit a short stem and swept bars and the steering will be as lively as if you still had the 71 degree angle.