The Mercedes-Benz 450SL uses a P21/5W bulb size for the front side marker and a 12821 bulb size for the rear side marker. The replacement chart covers vehicle production spanning 1973 to 1980 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 Mercedes-Benz 450SL uses two distinct bulb types across its front and rear side marker positions for the 1973-1980 model years, with each position requiring a different bulb entirely.
The P21/5W is a dual-filament bulb operating at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base, and it conforms to ECE standards used across European and worldwide markets.
The BAY15d base features offset pins, which prevents incorrect single-filament bulbs from being seated in the socket, making it a position-specific fitment that is not interchangeable with single-filament bayonet types.
The front side marker position on the 450SL calls for the P21/5W across all eight model years covered in the specifications.
The 12821 is a miniature wedge-style bulb of the T10 form factor, operating at 12V and 5W, with a W2.1x9.5d wedge base that requires no separate locking mechanism as the base itself provides retention through friction.
The rear side marker position on the 450SL uses the 12821 across all eight model years covered in the specifications.
The P21/5W and 12821 are not interchangeable with each other, as they differ in base type, filament count, wattage output, and physical form factor.
The 12821 is a Philips part number designation, meaning equivalent bulbs from other manufacturers are identified by different part numbers despite sharing the same T10 wedge specification.
The dual-filament nature of the P21/5W means the front side marker housing on the 450SL is wired to support two independent circuits through a single bulb, a configuration that would be incompatible with a single-filament replacement.
The W2.1x9.5d base of the 12821 is a standardized wedge specification, allowing cross-brand substitution as long as the replacement matches the T10 form factor and 12V, 5W rating.