1978
Is this a guess, or are you able to narrow it down that precisely?
1978 works for me.
We are calling it an educated guess. Using the process of elimination. Yes in 1979 the Letter A appeared on the kettles with this exact vent style. I have a Chocolate 22 with this same vent style. No date code.
Breakdown of the estimation:
1974 Kettles still had the BAR-B-Q-KETTLE vent and small bowl tabbed daisy vents and the old style all plastic wheels and thin whitewalls. The series of changes and transitions happened in this order, 1. The daisy vents were changed to the large dual tabs. (Presumably 1975 but not confirmed) 2. The wheels were changed to the modern "WEBER" whitewalls (Possibly in 1976, again not confirmed). 3. The lid vent was changed from BAR-B-Q-KETTLE to the "engraved" or "etched" version of the WEBER logo that has the elongated 'b' to accommodate the vent rivet. This lid vent still has the small turn tab. Presumably 1976 but not confirmed the vent may have changed to engraved in 1977) -There's not an abundance of kettles with this vent style out there. The last change (#4) before adding the date codes was to the lid vent updated to the modern style with the raised WEBER logo, wide turn tab and "MADE IN U.S.A" added below the PAT # 3538906. There have been examples of Lime Green and Yellow kettles with this vent style. Lime and yellow were factually available in 1977. There are no known dark blue or dark green MBH's with the "engraved" Weber logo
or the raised logo with no date code.
Those colors were introduced in 1979. Lime and Yellow were gone in 1979. That is how I narrow it down to 1978 or Autumn 1977-Summer 1978 (1978 model year)
Just like the earliest A codes were most likely made in 4th quarter 1978 in time for the '79 catalog. T-79.
EDIT: @dazzo has helped me to quadrilliontuple check my research.... (see strikethrough's_