I've wondered this for years, and I believe there was no finish on the oil caps. I think they were supplied bare steel.
Most parts that could be easily painted were - and from July 26, 1926 there's a document saying the breather cap was being painted black enamel for the 1927 model year: http://www.mtfca.com/encyclo/P-R.htm#paint3
Don't know if that was a change from before - should be documented on the drawing or the change card.
I purchased a dozen NOS oil caps years ago - all bare metal.