Dating abyss is right! I'm always studying and learning and correcting. We do need catalogs for sure.
Wheels (1970s)
Plastic rims with rubber treads and thick whitewalls: Very early 1970s, more than likely 1970-1971.
Early all plastic wheels, including treads and thin whitewalls. 1972-1975
Modern plastic wheels (used today) I think were introduced during 1975. Kettles from 1974 had small bowl tabs, I have a 22.5 Bar-B-Q-Kettle lid vent with large bowl tabs but the old style plastic wheels. I'm thinking mine is a '75. I've seen others with the same setup, but with newer wheels.
Ash Catchers (1970s)
thin/flat edge twist ash pans: Mid 1960s-1979
rolled edge twist pans: Changed during 1979 and issued through 1986. "A" code kettles have been seen with both types. My red 22 has the flat edge, Marty G has a black 18 with the rolled edge pan.
Now: Lid Vents, lets make it even more interesting...
Patent Pending.... ok its well documented that the patent number of 3538906 was granted on 11/10/1970. Well.......... The 1972 catalog, lists the 18 and 22.5 kettles as "patent pending" on the pages featuring those kettles. Why? Don't have a clue! The 1973 catalog gives the patent number....
It's got me thinking, we do see an awful lot of Patent Pending kettles for such a short window of time ('69-'70). Is it possible that Weber took two years to revise the lid vents with the patent number? Did Weber have a stock pile of Patent Pending lid vents made not knowing at the time when they would be granted the patent and not waste good materials? If so, that would account for the wide range of wheel styles we've seen on Patent Pending kettles. Sorry to turn this into a dork-off, but to me, studying these kettles is like watching your favorite movie over and over and catching something you missed the last time you watched.