Plymouth Custom Back Up Light Bulb Size

The Plymouth Custom back up light requires bulb size P21W or 1073 depending on the specific application. Data for the bulb sizes covers the range from 1958 to 1961.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1959-1961	P21W
1958	1073

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. Lower wattage variant of 1156. 12V, 13W.	1073

Other Locations

Additional Notes

The Plymouth Custom backup light specifications cover model years 1958 through 1961, spanning a period when backup lights were becoming more standardized across the North American automotive industry.
Two distinct bulb types appear across these model years: the P21W, used from 1959 through 1961, and the 1073, used in 1958.
The P21W is an ECE-standard bulb, meaning its specifications originate from European and international regulatory frameworks rather than North American ones.
The 1073 is an ANSI-standard bulb, placing it within the US and Canadian standardization system that governed most domestic automotive lighting of the era.
Both bulb types share the BA15s bayonet base, a single-contact base design that physically fits the same socket, which is a relevant factor when considering cross-compatibility between the two types.
Despite sharing the same base type, the P21W operates at 21 watts while the 1073 operates at 13 watts, a difference of 8 watts that affects brightness output and electrical load on the vehicle's circuit.
Both bulbs operate on a 12-volt system, consistent with the 12V electrical architecture that became standard in American passenger vehicles during the late 1950s.
The 1073 is described as a lower wattage variant of the 1156 bulb, indicating it belongs to the same bulb family but draws less power, which has implications for direct substitution with the more common 1156.
The shared BA15s base between the P21W and the 1073 does not make them interchangeable without consideration of the wattage difference, as substituting a higher-wattage bulb can stress original wiring and sockets designed for a lower load.
Both the P21W and the 1073 are single-filament bulbs, a construction type suited to on/off signalling functions such as backup lights, which do not require the dual-intensity operation of a two-filament design.