You do not have a "pre-production" model.
You have a mid to late 80s/early 90s model. It seems there was a period of time when the vents did not get date codes. I bet the handle has a single screw.
The door is aluminum. WSM doors that have stippling or a pattern are aluminum. Very early WSMs have steel
legs and smooth steel door.
The handle on the door is not wood either. It's some kind of plastic and was on all WSMs until less than 10 years ago.
There are two known WSMs with a "B" date code. The older of those two has a wood handle with 2 rivets, the other a single screw.
Edit:
If you have a pre-production WSM, it would have to be older than the oldest known WSM, and it therefore would have a wood handle with two rivets also.