Somewhere I have a group of Camillus S Cards sent to me by Tom Williams on the cusp of Camillus' closing and shortly thereafter. As I recall this knife was made for Schrade at the Camillus plant (both essentially Baer owned at that point). And they were made for a SFO customer but I don't remember which one. Yes it is likely stainless as the blade blanks are common with it's parent pattern the 897UH, the first Uncle Henry stockman circa 1966-67. I agree I am seeing surface debris, not patina and rust.
All Uncle Henry knives, folding and fixed, were not always stainless. A few of the fixed were originally carbon steel and, if they remained in the lineup such as the 153UH, switched to stainless at a later date.
Not all stainless blades are marked with the "+". All blades marked with the "+" are stainless.
Most Old Timers were 1095 Carbon steel but not all. Only a few that were stainless were so marked, usually in full text or by "SS". After circa 2002, several Old Timer patterns, folding and fixed, were changed over to stainless without being so marked or noted in ads, catalogs or packaging, or on the tangs.
As for the Swinden Key, it was for efficiency in manufacturing. And owning hundreds, having handled thousands, people's bad experiences with wobble and loose blades is overblown. By and large the key system works well. And did from 1958 through 2004 or the company would have ceased to use it. We are talking many millions of knives covered by a very liberal warranty policy. I have three dozen of this pattern under the parent tang stamp (897UH) and have used it as my EDC for many, many years and have hardly ever seen a problem develop. I do have dozens of traditional pinned knives which have some blade play from worn pins. But if you have one with Swinden construction and know what you are doing, you can disassemble it for blade replacement and put it back together.
Hope this helps.
Michael