I wasn't sure on the oil filter bracket: I'll have to spend some time with mine, looking for any paint traces. I would make sense that this would be natural metal, given it's assumed state in the assembly process.
No hard facts for you, but I've noticed that certain stamped sheet engine accessories seem to not take paint well; I've got brackets that I know were painted but have essentially zero paint on them.
They all seem to be stamped from sheet steel, non-heat treated, like power-steering pump brackets; alternator tension arms; york A/C pump braces, coil brackets; dipstick tube supports; that sort of thing.
Maybe some lubricant is used in the stamping/rolling process that doesn't fully wash off when they're painted. Obviously these parts don't get a lot of care in that area.
I find I can often strip all the paint of these sort of parts with soapy water and a scotchbrite pad, it almost falls off.
Add a little engine heat, oil, and time, and it might genuinely remove 100% of the paint.