Mercedes-Benz 450SE front side marker bulb uses P21/5W size while rear side marker bulb uses 12821 size. Replacement chart covers 1973 to 1976 production 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 450SE uses two distinct bulb types across its side marker positions for the 1973-1976 model years: the P21/5W at the front and the 12821 at the rear.
The P21/5W is an ECE-standard dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base.
The BAY15d base features offset pins, which physically prevents incorrect single-filament bulbs from seating in the socket, making filament-count matching a relevant fitment consideration.
The rear side marker position across all four model years calls for the 12821, a designation originating from Philips that corresponds to a T10 miniature wedge bulb rated at 12V and 5W.
The 12821 uses a W2.1x9.5d wedge base, which is a friction-fit design requiring no rotation during installation, unlike the bayonet-style base of the front marker bulb.
Because the front and rear side marker bulbs use fundamentally different base types, BAY15d and W2.1x9.5d respectively, the two positions are not interchangeable with each other.
The P21/5W designation follows ECE naming conventions, where "P" denotes a single-contact or dual-contact prefocus type, "21" refers to the higher wattage filament, and "5W" refers to the lower wattage filament.
The front side marker specification for the 450SE calls for a dual-filament bulb, meaning a single-filament replacement of the same base type would not satisfy the original specification.