Hi
In my opinion it is hard to tell exactly who the manufacturer could have been, however it is definitely a Japanese manufactured bike from the early to mid 1970's. From the previous reply, it could possibly be a Bridgestone or Kabuki bike. Anyhoo, it is a good strong workhorse bike and you should have no problem disassembling, cleaning and tuning this bike, or for that matter, getting replacement parts - all standard english threads. Be interesting what other replies will be, to see if someone can better identify it. Good luck.