S/N: M9K22505. After searching for answers for hours on end, I've convinced myself that I have a 1979 Shogun 400. F/R derailleurs are Altus LT and the crank is Shimano Tourney. I got it with Shimano 600 brakes, which I suspect had been changed out for the (what seems to be) original Tourney brakes. More details and pictures to come later...



Edit 1 - Tange 3 stamped on the back of the steerer tube.

also, added photos: