Hi Larry, enjoying your set up? The really old tapes (many of the 2 tracks for sure ) have an acetate base. The color of the oxide will actually vary with the same manufacturer over time but the material that it's on is pretty significant. The easy way to tell if a tape is acetate is to hold the reel up to a light. If you look through the windage holes, with an acetate tape, you can see some light but with all other bases, you can't ( there are a few exceptions). Acetate is actually the same stock that early film was made on and the early tape manufacturers (including Ampex) used the same stock to make recording tape. The problem with acetate is that it doesn't age well. It has a tendency to become brittle, much to the dismay of many silent film buffs. I just bought an old Verve, Oscar Peterson Trio "A Jazz Portrait of Frank Sinatra". It was made by Ampex and is on acetate. The sonics are excellent, in between the splices that is. There must be over 15 splices that are pretty well done and I've added tons of leader to both ends, but I broke the tape 2 times just trying to give it a first listen. I have a number of acetate tapes and going on sonics alone, you'd think that it would be the backing of choice. I think that it's more like a timing thing. I think the early reel to reel user was just like us. That's why you see so many demonstration tapes and "sonic spectaculars" with trains whizzing past and the poor little Bikini islands getting blown to smithereens over and over. They were more like statement products than what every kid was hoping to see under the xmas tree.
I have a feeling that the different record labels had a choice of the tape that was used (or made the choice with their wallets) because some tapes I have of the same time period, and manufactured by Ampex at the same plant, were recorded on different formulas of tape.
I don't know if you've already traveled down the Sticky Shed Syndrome path but that is another very important tape consideration. In the mid 70's, because of the oil embargo (remember that wonderful little panic) Ampex was forced to change the binder that glues the oxide to it's backing. They made a real bad choice with their 456 tape (as did a few other manufacturers) so, depending on the record company, some of the tapes of this era may have SSS. For more on that, check out Richard Hess's site that's linked in my sticky at the top of the General forum. Agfa had a similar problem when they decided to lower the pollution from their plants in Germany.
So the tape that Ampex spooled for the different recording companies could have changed in mid issue with some releases. It could be because of the record company, or it could be because of Ampex.
Lots of the commercial issues were made on 1 mil or less. That's why I avoid the twofers.