What is that oil filler cap from? It doesn't look like any stock A series cap that I have seen lately. It looks, sort of,.... billet

Most likely. To be double-sure, you can do a compression test and a cooling system pressure test. We worked on a Camry last year with milkshake like that and it turned out to be the water passage in the intake manifold. Got the engine torn 1/2 down (but the head was still on) when I decided to do the radiator pressure test a second time. The first time it was just air leak I could hear seemingly in the crankcase but the second time (after I got the manifolds off) I refilled the radiator, then I saw coolant coming out the intake ... whew, I was glad I didn't pull the head. All it took to fix was a $2 gasket.

Contrary to popular opinion, A-series rarely warp the heads. So even if the gasket is blown, a careful deck prep (scrape and clean) and straightedge check is usually all that's needed. Do be sure to have it milled flat if it's out spec, but that's not usually the case. They can take a moderate overheating and just blow the gasket.