IIRC Girling calipers from the late 60s/early 70s were mostly plated gold, not sure if this was zinc or cad. I send my business to PMB Performance in Utah for yellow zinc, no affiliation but I've been happy with their work. They are brake specialists, actually, but work w/ one plater in Salt Lake who seems to know how to decontaminate & prep these parts properly. (When the plater doesn't do it right those dreaded white spots show up way too quickly!) I usually have the fasteners done in black zinc at the same time.