Some tidbits I found about deciphering Mercedes part numbers. Mercedes usually uses this format 123 820 41 64 (3 digits, 3 digits, 2 digits, and 2 digits). This is the p/n for a Driver's side tail light lens for a 123.

-The first three digits (123 820 41 64) are the chassis the part was first used on. This number can also be the engine number as well as other numbers.

-The second set of three digits (123 820 41 64) refers to which Group the part belongs to (ie. brakes, suspension, etc...). 820 refers to Body Electrical. This number is also used in the Factory Service Manual, group 82 section with all the body electrical service info.

-The first set of two digits (123 820 41 64) refers to which version of the part it is. Meaning there could be several versions or runs of one, part each with a slight modification. We have the 41 version of the lens.

-The last two numbers (123 820 41 64)are the part number in the master listing. In the number above we have the 64th part in the Body Electrical Group.

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