aged brass typically takes on a brown appearance but rarely turns RED... copper of course has a red hue. Incidentally bronze also takes on some interesting hues when it ages.
it definitely COULD be brass in the Yuma and ISYHCANL versions. I DO believe the MoM exhibit hero is copper rails. Aged brass just won't look that red without a paint.
Occam's razor tells me that the exhibit prop is copper with a wax coating or clear coat to prevent oxidation.
the ISYHCANL I would have until recently sworn was brass but
BRRogers brought that into question for me and raised reasonable doubt. The Yuma I believe is brass L-stock and have seen nothing to disprove that or raise question to it.
so my personal theory? Yuma: brass. ISYHCANL: modified box and either kept the brass or moved to copper... Cave: copper