Rod,
Your #326 got me looking at Horns again...I found it, it's for the 1957 Passenger cars.
I also found in my "1956/1957 Chevrolet Heavy Duty Truck Parts Catalog and Dealer Price Schedule" dated March 1, 1957 lists the following:
1957 Left side Lo note #327
1957 Right side Hi note #328
So, was there a mid year change???
The March 1, 1958 Book has the 1957 trucks as using the 345/346 Horns. It also shows that the 1957 and 1959 trucks both used the #346 HI note (accessory right side horn) and that 1958 used the #348 Hi note (accessory right side horn). So why did they drop the 348 after one year???
Mike B
