The Mercedes-Benz 450SLC 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 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 450SLC 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 conforming to ECE standards used in European and worldwide automotive applications.
The BAY15d base features offset pins that prevent incorrect insertion orientation, a design characteristic relevant to dual-filament bulbs where polarity and filament alignment are functionally significant.
The front side marker position on the 450SLC calls for the P21/5W across all eight model years covered in the specifications.
The rear side marker position on the 450SLC uses the 12821, a Philips-designated miniature wedge bulb of the T10 type, fitted with a W2.1x9.5d base and rated at 12V, 5W.
The T10 wedge bulb relies on spring tension from the socket rather than a locking base mechanism, which distinguishes its fitment behavior from the bayonet-style P21/5W used at the front.
The P21/5W and 12821 are not interchangeable, as they differ in base type, filament count, wattage, and physical form factor.
Substituting a single-filament bulb in a socket designed for the dual-filament P21/5W would result in only one circuit function being served, leaving the other non-operational.