To me, it looks like your top vent says Bar B Kettle over the rivet, which should put it '76 or earlier. The question becomes is there the patent number on it, does it say patent pending, or is there no reference of a patent. That would narrow things down a bit.
I can't be sure, because the wheels are mostly in the shadows & the resolution isn't very high, but the wheels look like they have that seam that runs along the circumference through the center of the tread width. So, likely the '73-76 wheels (which is kind of what I get the imprssion of when I enlarge the low resolution photo).
Better pic of the top vent &, depending on what is there, maybe whether the bottom vent has tabs like the top vent or tabs that are significantly larger & you'll get it pinned down fairly well.