I did a little more research and here's what I found...
1955/56 Left side Lo note #759 12v
1955/56 Right side Hi note #760 12v
1957 Left side Lo note #345
1957 Right side Hi note #346
1958 Left side Lo note #347
1958 Right side Hi note #348
Source;
1956-58 Heavy Duty Truck and Special Equipment Cataloge 3-1-58
Later parts books start combining years to reduce inventory (if it "fit"). A perfect example is my 1960 truck parts book shows #346 as being correct on ALL 57-59 Right side Hi note.
Again, as far as I know the Sphire/Dome cover was last used in 1956, the '57 horn is still a Sea Shell, but it has a cylinder type cover.
These horns were used in all of the GM cars and trucks with the only difference being the part number to ID the bracket.
Mike B
