Thought you had me there for a minute, but he doesn't put it on until after this sequence, when he's in his X Wing. His hand hadn't been shot yet in the scene these pics are from.
In response to your earlier question, I don't think your information is correct.
The screws used in the Yuma box could have come from literally anywhere, and there were certainly steel and aluminum slotted screws around in the early 80s, and long before that. (And zinc has been in use as a protective coating for a long long time)
I suppose if you're asking whether the regular Joe could go to the local hardware store and purchase any screw under the sun, in whatever material he could imagine, in 1982-3, I can't give you a definitive answer, without knowing where.
But, I can tell you there was likely no shortage of fasteners in whatever shop the prop was constructed.
They could very well be painted or blackened brass, and just as easily blackened steel. My guess is probably not aluminum, but it can't be ruled out.