As I was playing around with my Dawes frame today, and was wondering about the tubing (Reynolds 531 DB?), I took a similar-sized Union frame from the attic that I thought I knew was hi-ten and weighed both. Sure enough, they were about 400 grams apart. With the steel headset on the Dawes and the Shimano 600 alloy headset on the Union, the difference is probably even bigger.
I had a 26.8 mm seat post in the Dawes, and just out of curiosity I stuck it in the Union. It fit perfectly. What's up with that? 26.8 is not supposed to fit a hi-ten frame, is it?
It's probably a hi-tensile frame with a Tange #5 CrMo seat tube. This was a fairly common ploy by some manufacturers to get a CrMo sticker on an netry level frame. If you looked closely at the tubing decal, it disclosed that only the seat tube was Tange #5.
Thanks for your reply, T-Mar. It must be something like that. This Union frame is a bit of a mystery. It is supposedly a Union Sapporo (says Union), but I've never seen another one in this color. There's no tubing decal (or any other for that matter), just a head badge. It is a nicely built frame, with wrap-around seat stays, and best of all: tall and short with a fairly long wheelbase, which should make a comfortable tourer for yours truly..
There are a couple of other possibilities. If the owner wanted a micro-adjust post, they may have reamed it to 26.8mm, which is a very common size. Micro-adjust posts are available in the smaller sizes typical of hi-tensile steels but can sometimes be hard to find. Also, we can't discard that it may be a ultra-lightweight hi-tensile seat tube, even though I've never seen one that thin. Low 26mm range are fairly common on some club racers and the thinnest I'm aware of is Columbus Zeta, which used a 26.6mm post.
I'm not very well-versed on steel tubing. I suspect the frame was made by Sakai, so it could be Tange of some sort. Union only did the sturdy bread-and-butter stuff themselves; the road bike frames were made by subcontractors. The high-end "Rini Wagtmans" model was made by Gianni Motta, and the low-end models with stamped dropouts were called Union-Sakai. The Sapporo is somewhere in between. Originally equiped with 600 Arabesque. Dropouts are Shimano. Serial number is M1B09266.