I finally got to dig up both frames and compare the two. One more mystery solved!

Thanks for the pointer, [MENTION=205648]Hummer[/MENTION]. Both frames are identical. It is safe to say that the faux Bontekoe is a repainted Union Sapporo. These were sold in the late seventies and early eighties and were originally fitted with Shimano 600 arabesque; the original headsets are still present in both frames. Union also had a cheaper model in the lineup in those days with stamped dropouts called Union Sakai, so I'm assuming these came from the same source. The Sapporo carried a Hi-Ten sticker, but some sources suggest there's some Cro-Mo involved, if only the seat tube. The 26.8 seat post diameter certainly backs that up.

I am still curious as to whether these were made especially for Union, or if these were also branded as Sakai or Sekai (or maybe something else again) for other markets.