I think the answer to your question has been pretty well covered. And just to repeat, the correct term for pre-70 fresh air intake air cleaners is Cowl Plenum air cleaners (2yrs ago Chuck Hanson brought a 69 to CB07 with a CP air cleaner, supurb car!!!). The reason is because the air actually comes in from the vent area of the cowl just in front of and below the windshield. At one time, these air cleaners were available from Chev Parts Dept.
Factory installed??? That's always been debatable. But for sure, it was a part that you could buy from Chev. And for sure, the first time that it was available (and it WAS installed at the factory) as a genuine, installed part, was on the 1963 RPO Z11. The Z11 was the complete pkg (ONLY came one way) with a 427 (essentially a stroked 409), alum front sheet metal, bumpers, etc, 4sp, posi, no radio, heater and in a plain 2dr Impala ONLY (NON-SS) with dog dish hub caps.
The Z11 Impala actually arrived at the dealer with the Cowl Plenum air cleaner installed. It is also debatable as to how many of these Z11s were assembly line built (components were available from the parts dept). The numbers range between 47 and 57 cars actually built at the assembly plant. We may never know for sure the exact quantity.
There MAY be about 10-12 DOCUMENTED Z11s in existance today. One was recently discovered, bought and is presently undergoing restoration now.