Hey Steve,
Welcome to the forum.
I was looking at my listing of 3M reel to reel tape, and the numbers jump from 250 to 265.
250 was a pretty common audio tape and I have a number of reels (some sticky, some not so much). Some are 1/4" and some are 1/2" but they're all labeled 250.
In my list, the 265 is 1.05mil and is specified for "digital" (computers or early digital audio recorders I will guess since it was produced in 1977).
Now, the reason I mentioned the tape width before is that small number changes 141-142,154-156,295-296, for tapes introduced the same year, seem to indicate either differences in thickness and sometimes (but rarely) oxide type.
So, I would guess that it's "most likely" that it was the same tape as 265 but 2mil.
If you're super curious, you can check with the guys over on the Ampex list. You'd be amazed at the stuff those guys know (Bill Lunde (sp?) hangs there and he was in charge of some of the 3M tape formulations ).
