GMC S370 front and rear turn signal bulbs utilize 1073 and 1129 sizes. The replacement chart covers 1951 through 1957 production years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The GMC S370 uses two distinct bulb types across its model years for both front and rear turn signals: the 1073 and the 1129.
The 1073 is a single-filament bulb rated at 12V and 13W, fitted with a BA15s bayonet base, and is classified as a lower wattage variant of the 1156 under the ANSI standard applicable in the US and Canada.
The 1129 is a dual-filament bulb rated at 12V with a 21/5W output, fitted with a BAY15d bayonet base, and is classified as a variant of the 1157 under the same ANSI standard.
The BA15s base of the 1073 and the BAY15d base of the 1129 are not interchangeable. The BAY15d base features offset pins to prevent incorrect installation, while the BA15s base has a single contact and symmetrical pins.
For the GMC S370, the 1073 bulb is specified for both the front and rear turn signals in the 1956-1957 model years.
The 1129 bulb is specified for both the front and rear turn signals across the 1951-1955 model years.
The front and rear turn signal positions share identical bulb specifications in every model year covered, meaning no positional difference exists between front and rear fitment requirements.
The transition from the dual-filament 1129 to the single-filament 1073 between the 1955 and 1956 model years represents a change in filament configuration, which carries base incompatibility implications for direct substitution.