I have a Pre 25, model of 1955 that is nickle, I ask Roy Jinks, S&W Historian, if it was original and he told me it left the factory blued. It was a 5 screw and pretty and the price was right so I bought it anyway. If you are a member of the SWCA you can use the forum at the bottom and ask Roy Jinks. But you have to be a member, one of the many perks. I have never heard of a Nickle M-1955, but with S&W never say never. You could ask for a letter, I think they cost $100 for a non-member.
SWCA 892