It was zinc plating.
To be more accurate, it was closer to "dull zinc" plated than "clear zinc" (shiny) plated.
The sockets should be knocked out before having the housings plated. I use a thin wooden block to tap them out.
I use a 1/2 inch drive, deep wall socket to get the lamp socket out with "gentle force". (Sorry, I don't remember the exact size. I use one that fits the outer edge of the lamp socket.)
If you're trying to make originals better without re-plating, try soaking in rust911 or evaporust. I'm liking the rust911 product better than evaporust these days and it's much cheaper.
If you have any rust spots on the housing, the use of Evaporust "may" make these spots more obvious.
(One "trick" I did on my 1966 daily driver was to spray paint the inside of the housing with chrome paint. It made the tail and stop light brighter.)
Jim