Well, so I and a few others think that the steel stems should just be called Yeti stems (since not only FTW made them) the true FTW stems were the aluminum ones made before the ATAC came out. Those are much rarer and AFAIK all were made by FTW.
It's conceivable that Yeti made some early steel stems to fit BMX headtubes. Not sure I can tell 0.7mm difference on a small pic. So, unless someone comes up with a Yeti made stem with a 21.5mm quill I think they were all 1" and came out around late mid to late 87 maybe. Hard to say on that as older bikes still in service could have gotten newer stems, etc. I guess seeing team riders with those stems would be a good indicator since they were usually on the latest and greatest.
FRO XR341 had an aluminum FTW stem, somehow I feel it was added later, but there is not way of knowing.