I think the #'s you are seeing, which are raised and cast in, are the casting part numbers, there should be a plate riveted to the curved part of the dash near the clutch pedal with the S/N and model no. stamped into it. 880 serial numbers started at 60505 and ended at 135054. Other posters who are sharper than me can tell you some other ways to tell the year of mfg. if the S/N plate is missing, should be a date  the casting was made near the part no. on the frame, tramsmission, and engine block. This will get within a few months of the assembly date.
Harry