Mercury Montclair Back Up Light Bulb Size

The Mercury Montclair back up light requires 1129, 1141, or P21W bulb sizes depending on the specific application. Available information covers the period from 1955 through 1968.

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
1967-1968	P21W
1964-1966	1141
1958-1960	P21W
1956-1957	1141
1955	1129

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. 12V, 21W.	1141
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. Variant of 1157. 12V, 21/5W.	1129

Other Locations

Additional Notes

The Mercury Montclair back up light specifications span model years 1955 through 1968, with two ANSI bulb types and one ECE bulb type represented across that range.
The P21W is an ECE-standard single-filament signalling bulb rated at 12V and 21W, fitted with a BA15s bayonet base. It is the designated back up light bulb for the Mercury Montclair in 1958-1960 and 1967-1968.
The 1141 is an ANSI-standard single-filament bulb also rated at 12V and 21W, sharing the BA15s bayonet base with the P21W. It is specified for the Montclair in 1956-1957 and 1964-1966.
Because the P21W and 1141 share the same BA15s bayonet base and identical voltage and wattage ratings, they are physically and electrically equivalent in most respects, though they originate from different standardization bodies.
The 1955 Mercury Montclair uses the 1129, an ANSI-standard dual-filament bulb rated at 12V with a 21W primary filament and a 5W secondary filament, fitted with a BAY15d bayonet base.
The BAY15d base of the 1129 is offset-pin, meaning it is not interchangeable with the BA15s base used by both the 1141 and the P21W. Substituting one for the other without an adapter is not physically possible.
The 1129 is noted as a variant of the 1157, a widely recognized dual-filament automotive bulb. The shared base type and dual-filament construction reflect a design intended to serve combination signalling functions.
The back up light circuit on the 1955 Montclair draws on only one filament of the dual-filament 1129, as back up lights are single-function applications. The secondary 5W filament would remain unused in that role.
The shift from the 1129 in 1955 to the 1141 in 1956 represents a change in base type from BAY15d to BA15s, which affects socket compatibility between those two model years.
The 1141 and P21W appear across the majority of the listed model years, making the BA15s bayonet base the predominant socket type in the Mercury Montclair back up light application across this specification set.