Take a look at the images at that link, for the C codes. Steel door, steel legs (rusty.) It's unlikely there would be one similarly as old but made with aluminum.
TVWB's age guide is excellent, and more detailed than ours. But I think their assumption re: manufacturing errors regarding why some WSMs have no date code is just that, a guess. What we do know is that Weber is (in)famous for using standardized parts where possible.
Companies like Weber rarely employ archivists, so we'll never know the real story. That being said, it's a safe bet they had a batch of unstamped vents they ran through at some point to save money during the late '80s/early '90s economic downturn, or had excess inventory of them, or whatever.
Here are some questions I have. Would, or did, B, C etc WSMs have handles with The One The Only on them? If so, that handle on yours is "too new." The door may provide a better clue. What years were they diamond engraved like that?