That's not a nickel plated gun. A nickel plated 75 will have black controls and a non-ambi safety.
You have a stainless 75B. You underpaid by at least $150 
Nice snag. I own both a polished and a brushed stainless 75B and I still would have picked that up for the price you paid.
Upon much closer examination I believe you're correct and I'm in error. Inside the frame and slide, on the rails, on the shaft of the slide stop, in the slide serrations, and in the grooves on the top rib there is what appears to be the copper plating that is usually the base for a nickel plate. I've never seen this before on a stainless gun. And the "feel" and "color" is different from my other stainless guns. I have several stainless handguns and several nickel-plated ones. I felt I was quite familiar with the differences, but perhaps I'm learning something new.
FWIW, I thought it was gloss stainless when I first saw it in the display case. It wasn't till after I had disassembled it that I convinced myself it was nickel-plated. With it's ambi safety, upswept beavertail, and deeply curved trigger the frame is essentially identical to my LE, which was made in 2009. As I said, this one was made in 2008. However, the slide serrations are considerably different from (and more aggressive than) the serrations in either my LE or polycoat.
Whatever it proves to be, I'm delighted with my find. Thanks for the corrections and comments.
BTW, it shoots well. But, I'd forgotten how a 75B begins as a diamond in the rough.