The Mercedes-Benz 450SEL uses a P21/5W bulb size for front side marker applications and a 12821 bulb size for rear side marker positions. The replacement chart covers 1973 to 1980 model 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 Mercedes-Benz 450SEL uses two distinct bulb types across its front and rear side marker positions for the 1973-1980 model years.
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. It conforms to ECE standards used in European and worldwide automotive lighting applications.
The front side marker position on the 450SEL calls for the P21/5W across all 1973-1980 model years.
The BAY15d bayonet base of the P21/5W features offset pins, which physically prevent the bulb from being inserted in the wrong orientation. This ensures the two filaments are connected to the correct circuits.
The rear side marker position on the 450SEL uses the 12821, a Philips part number designating a miniature wedge-type bulb, for all 1973-1980 model years.
The 12821 is a single-filament bulb rated at 12V and 5W, fitted with a W2.1x9.5d wedge base, also referenced as a T10 form factor. It carries no bayonet or pin-type retention mechanism, relying instead on the friction of the wedge base within its socket.
The P21/5W and the 12821 are not interchangeable. Their base types, filament counts, and wattage ratings are fundamentally different, and substituting one for the other is not supported by these specifications.
The consistent use of the P21/5W across all listed front side marker years indicates a single fitment requirement with no variation throughout the entire 1973-1980 production span of the 450SEL.
Similarly, the 12821 applies without variation to all listed rear side marker years, reflecting a uniform rear fitment requirement across the same production span.
The higher wattage of the P21/5W front bulb relative to the 12821 rear bulb reflects a functional difference, as the front position uses a dual-filament design capable of serving both a running light and a signalling function from a single unit.