The GMC S300 front turn signal requires a 1073 or 1129 bulb size, while the rear turn signal uses a 1034 or 1129 bulb size. The replacement chart covers 1955 to 1957 production years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1073 bulb is an ANSI-standardized single-filament type rated at 12V and 13W, fitted with a BA15s bayonet base. It functions as a lower-wattage variant of the widely used 1156 bulb.
The GMC S300 uses the 1073 bulb for the front turn signal position in the 1956-1957 model years.
The BA15s base found on the 1073 is a single-contact bayonet fitting, meaning it is not interchangeable with dual-contact bayonet bases such as the BAY15d used on other bulbs in these specifications.
Both the 1034 and 1129 bulbs share the BAY15d dual-contact bayonet base and are each described as variants of the 1157, operating at 12V with a dual-filament output of 21/5W.
Despite sharing the same base type and wattage rating, the 1034 and 1129 carry distinct ANSI designations, which indicates differences in their precise filament or optical characteristics that affect fitment suitability.
The 1129 bulb serves both the front and rear turn signal positions on the GMC S300 for the 1955 model year, making it the only bulb in these specifications used across multiple positions within the same model year.
The 1034 bulb is designated for the rear turn signal position on the S300 for the 1956-1957 model years.
The front turn signal position on the S300 transitions from the dual-filament 1129 in 1955 to the single-filament 1073 in 1956, representing a change in filament configuration between those model years.
All three bulb types listed, the 1073, 1129, and 1034, conform to ANSI standards applicable in both the United States and Canada.
The dual-filament design of the 1034 and 1129 bulbs allows a single lamp housing to serve more than one electrical function, such as combining turn signal and running light duties, which is a common application for BAY15d-based bulbs in automotive use.