The GMC M340 front and rear turn signal bulbs utilize either 1073 or 1129 size specifications. The replacement chart covers 1955 to 1956 production years with corresponding bulb size data.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The GMC M340 uses two distinct bulb types across its 1955-1956 model years for turn signal applications: the 1073 and the 1129, both of which conform to ANSI standards recognized in the United States and Canada.
The 1073 bulb is a single-filament type operating at 12 volts and 13 watts, fitted with a BA15s bayonet base. It functions as a lower wattage variant of the more widely known 1156 bulb.
The 1129 bulb is a dual-filament type rated at 12 volts with a split wattage of 21 watts and 5 watts, mounted on a BAY15d bayonet base. It is a variant of the 1157 bulb.
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 uses a single-contact, symmetrical pin arrangement.
The dual-filament design of the 1129 serves two independent circuits from a single bulb, which is a characteristic associated with combined stop and turn signal functions in automotive lighting systems.
The 1956 GMC M340 uses the 1073 bulb in both the front and rear turn signal positions, replacing the 1129 specified for the 1955 model year across those same positions.
Because the 1955 model uses the 1129 and the 1956 model uses the 1073, the two model years require different socket types due to their incompatible bayonet base configurations.