i know on some suzukis around that year they changed part numbers for the frames halfway through the model run and this was usually due to an increase in footpeg pivot pin size (8-10mm) therefore more than one frame part number will come up for one model. I cant say for sure that is the case with the 77 125 though.
for 77 2 frame numbers show up.
Part # = 41100-41302-019
Part Description = FRAME
Model Count = 1
RM125B 1977
and
Part # = 41100-41303-019
Part Description = FRAME
Model Count = 3
RM100C 1978
RM125B 1977
RM125C 1978
several foot peg part numbers show up too