I just purchased a 27” Bridgestone road bike and Im wondering what model it might be. From the serial number and a little researching I’ve learned it’s probably a 1980. The decals are intact but no reference to a model number. The rear derailleur is a Shimano Altus C10 with a six speed free wheel and friction shifters mounted to the stem. An article I read referenced RB-1, RB-2, and RB-3 among others as possible models. How would I determine what model bike I purchased?

Thanks.