To the best of my knowledge, brass screws were not used for gunmaking until the late '60s.
That's the 1960s, :: when the modern forign reproduction guns started selling.
I suspect some modern stylest got inventive and said "That would look and sell better if it had brass screws holding the patchbox on." ::
It is just a guess of mine, but because most of the wood screws were hand made, I suspect the gunsmiths felt that if they were going to go to all of that work to make them, they might as well do it using something that would have some strength and not get screwed up (pun intended) by the simple act of unscrewing the screws a few times.