How often are WSM double stamped?

When I went and picked up my WSM I got the other day I noticed its double stamped on the top vent. It not only has a C (I guess I'm officially part of the "C" club) but also a D on the same vent. How often does/did something like this happen? Overall it needs a new handle and some wire bush work and shine but the inside and outside are in good shape except for a couple small chips. Sat unused since 1999 which I'm sure contribute to the rust on the racks, door, and legs. I would say it sat way more than it was used because there isn't really any noticeable clue that bbq was ever cooked in it.