Copper or brass terminals on a distributor cap ?
Thanks.

My guess would be copper since brass is just copper with impurities added to the copper.

Brass is a copper, zinc alloy. Brass is more corrosion resistant than pure copper.

Delco-Remy always offered a marine application cap with brass contacts.

It depends on if you live in a wet climate where corrosion is a problem; just like a penny if corrosion builds up resistance goes up which could cause a problem that is why they use brass in high humidity areas. Since I live in NV it’s not a problem so I'd use copper, but that may not be the best choice where you live.