Large bowl tabs, 1975 is pretty solid. Late '74 at the earliest. Large lid tab, 1978. The last MBH kettles were 1980 "B" code.
There's BBQK small tabbed lid vent kettles with large bowl tabs. (most likely the late '74/1975 kettles)
There's "engraved" WEBER logo'd lid vents (small tab) with large bowl tabs (1976-1977 era)
Large lid vent tab with the raised WEBER logo and large bowl tabs....no code, 1978.