I'd think that it did leave the factory as a blued gun based on the marking on the butt. The "B" on the barrel doesn't look familiar to me but I haven't seen a lot to compare to. Here's a pic of a what is a lettered 38 M&P Nickel not nearly as early as the one you have posted. This one does show signs of refinishing also so I suppose none of this really helps give you a definitive answer. I tried
.
I should add that my 1899 with blue finish has no "B" marked on it anywhere.