Brass and bronze are often confused with each other, and are more often than not referred to as "copper alloys" these days. However, as brass is typically more malleable than bronze, it's often used for instruments and vases and such, while bronze is used when resistance to wear and corrosion is needed. I'm pretty sure that Triumph oil pumps are made from bronze. Personally, I've never seen an iron oil pump in a Triumph engine,, and I have actually seen a few.
Brass (soft) has very poor wearing properties and would not be any good for a pump. They appear to be made of Gun Metal (Red Brass in USA), which is a type of bronze; an alloy of copper, tin and zinc. Its typically around 88% copper, 8–10% tin, and 2–4% zinc. It cast very well and was originally used chiefly for making cannons. It was also widely used for valves and fittings on steam locomotives and steam boilers and is much harder, stronger, wear and corrosion resistant than brass.
I see it regularly working on steam locomotive valves and fittings regularly at a heritage railway, It is a different colour (more gold coloured than yellow) like brass and I am quite familiar working with it..
You guys know your stuff, unlike those silly nits at Triumph.