The design changed through the production run as perce says.
Brake pedal design, shift lever design, air caps on forks and casing protection loops on the frame. My ride 348 is 51M 10476 motor and frame and has the original style (folding) brake pedal tip, (folding) shift lever tip and no case protectors. Mine does not have the signature on the fibreglass under the seat. It also doesn't have the slotted line along the tank. My memory of the sequence was 76 has the MRR signature, 77 has a fairly plain tank and 78 has a gold/black slotted line along the sides of the tank.
I'm thinking that this probably means my ride 348 is 1977 but I'm happy to hear of other opinions.