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.