Would think that by the SLR era, top & bottom covers / plates (stamped sheet metal trim) were brass, but the bodies themselves were some sort of casting (aluminum or die-cast zinc alloy).
Not certain about late metal-clad cameras, such as the Canon AE-1 Program, but believe the trimmings would have been brass until the switch to plastic / carbon fiber bodies/casings.
Just my 2-cents' worth...
LF