The 1154 bulb is an ANSI-standard dual-filament type with a BAY15d bayonet base, rated for both 6V and 12V operation, and is noted as an older variant of the more widely recognized 1157 bulb.
The 1158 bulb shares the same ANSI-standard dual-filament construction and BAY15d bayonet base as the 1154, but is rated exclusively for 12V operation.
Both the 1154 and 1158 bulbs conform to ANSI standards recognized in the United States and Canada.
The BAY15d bayonet base features two offset pins, a design that physically prevents incorrect single-filament bulbs from being seated in a dual-filament socket.
The Plymouth Deluxe uses the 1154 bulb for both front and rear turn signals in 1951, while the 1158 is specified for both positions in 1950, meaning the two model years are not interchangeable in bulb selection despite sharing the same BAY15d base type.
Although the 1154 and 1158 share the BAY15d base and would physically fit the same socket, substituting one for the other without accounting for voltage compatibility could affect circuit performance, particularly given the 1154's dual-voltage rating versus the 1158's 12V-only rating.
The dual-filament construction of both bulb types serves the combined function of turn signaling and another lighting role, such as running or brake illumination, within a single housing.