Okay, I think it is a Bridgestone/Kabuki/C. Itoh Super Light. The seat tube is steel because it uses a quill seatpost. It uses a quill seatpost because the aluminum lugs are too stiff for a slot clamp. The bit that looks like a slot clamp is the mount for the center pull brake cable. My friend and I just finished building it back up. It's actually pretty light for a ~40 yo bike!
This was my first time really getting down and dirty with a vintage bike. I learned a lot. JIS square taper - wtf?