Usually below the "B" in "WEBER" - they can be hard to see, especially if there is a healthy coating of crud on the damper vent. See if maybe there is a code on one of the bottom damper vents.
That is a great looking classic Weber. I see the grills that the guys on the board find and get jealous, until I look at my 1991 Red Stainless Performer!!
I do like the old ones, and I like the red ones the best!
Congratulations on a great find!
Pat