As I said, Bill, old style, as in 100 or so years ago.
It was apparently done that way because it was faster and easier to plate the steel with copper, and then the copper with nickel.
As the technology progressed, copper underplating was slowly dropped.
That said, solvents, including Hoppes, have been known to attack nickel plating for decades.
It is not, however, a quick process, nor will it strip an entire gun quickly and surely.