Just did some digging through the '65, '66, and '67 FPCs.
Air cleaner lid, '64-'65-'66, chrome with 273-4bbl, without Clean Air Package: 2532 609
Air cleaner lid, '64-'65-'66, chrome with 273-4bbl, with Clean Air Package: 2432 616
Air cleaner base, '64-'65-'66, with 273-4bbl: 2658 437
The Clean Air Package version of the air cleaner lid has the fitting on the side for the hose from the crankcase breather. The non-CAP version of the air cleaner lid does not have the fitting, because there was no such hose (a bottom-vented crankcase breather was used on non-CAP vehicles).
CAP was mandatory for all vehicles sold in California, regardless of build plant, and optional on all vehicles sold elsewhere. I don't recall whether your '65 is a California(-sold) car or not.
So, no P/N differences (Dart vs. Valiant, year vs. year) to explain the indentation and slightly different crankcase breather hose nipple placement on one of your two lids. I'll run it by a guy I know who was an engineer for the company that supplied Chrysler with most of their air cleaner housings in that year range and see what he has to say; will report when I hear from him.
Also keep in mind these same air cleaner lids (with and without crankcase breather fitting but without chrome) were used on many '61-'69 slant-6s, so it's a solid bet that over the years a good number of lids have been chromed to "convert" them into 273-4bbl air cleaner lids. Given the extremely large volume of slant-6 installations using these lids, it's an equally solid bet that they were coming from more than one stamping source and minor variations that didn't affect fit or function (such as these what you're showing) would have been tolerated with no backtalk from Chrysler.
