OK this might sound weird, but here goes.

I'm trying to figure out the VIN Number of a car I sold a couple years back. It was a 1985 300D.

I found my old insurance records, and found the number listed on them. The Company lists the VIN Number as:
WDB123130FA215511
But when plugged into carfax or any other VIN information or decoder tool, it shows up as invalid.

After looking at the VIN numbers on similar model year cars, I found that those 6 digits in between B and F make no sense. After reading up on what the Mercedes VIN Number means (Still don't have a very good site for this, had to piece it together) I have made up the middle numbers to look something like this:
WDBAB33(XX)FA215511

I still cant figure out for the love of me what those two digits (XX) mean. I have tried combinations using the Mini-carfax look up (without paying, so i don't get any info, just whether it works or not) and found that "A5" and "C3" work.

Can anyone tell me what those 2 digits mean? And why more than one combination would work? I thought the last 6 Digits were unique to every vehicle.

Thanks in advance!

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I love my cars to death but my wallet sure doesn't.