I don't know how OLD you're talking about but I bought a stainless T/C frame in the early 90's, maybe even 1989. It is truly stainless steel. A few years ago I bought a stainless steel barrel on an auction site and it didn't fit my stainless frame. I called Thompson Center about it and they had me check numbers and markings on the lug and as it turned out, it was an old Armour Alloy barrel. The appearance of it was VERY different from my stainless frame....had a nice luster and slightly brighter color to it. In any case, in order to get the barrel to fit my stainless frame, I had to send it to T/C and have them fix a few things (larger diameter lug hole and possibly something with the locking system) all at no charge. When I got the barrel back, they even gave me a new hinge pin with it (no charge!). The barrel fit, locked up, and shot well, however, I was advised by T/C gunsmiths and other gunsmiths NOT to load the barrel with high pressure ammo. (The barrel was a 45 Colt and I load some hot loads in that caliber). I think the T/C people just said not to for liability reasons but the gunsmith that I know warned that it was NOT a true stainless steel barrel and he didn't know exactly how it was made, nor the true strength of it and that the finish on it was actually a zinc or nickel alloy plating of some sort, as I recall.
By the way, a rep from T/C told me that they ran armour alloy frames and barrels for a few years and then stopped the production of them. I don't know the exact year when they did this but they definitely weren't the original form of "stainless steel" guns made by them. I believe it was just a special run during the early 80's or something like that.
That's all I can tell you. I've since sold the barrel that I had, needless to say, it was a nice barrel and a lot of fun to shoot .410's out of it. It wasn't much of a 45 Colt shooter.
