That's a '70-'73 air cleaner; starting in '74 the snorkel was oval instead of rectangular.
From the factory, a '76 Slant-6 had an air cleaner service instruction decal on the side of the air cleaner housing, and sometimes a genuine-Mopar-parts decal—if whoever on the line was supposed to apply them wasn't absent, drunk, or high that day. The instruction decal shown here is a V8 item; the Slant-6 item was the same except for the filter element part number (2206376). I doubt the Slant-6 item is available as a reproduction, but if anyone has it, that'll be Jim Osborn Reproductions.
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