There would not be a "r" prefix in the serial number. Did you take that from under the barrel or from the underside of the frames grip butt?
Assuming the serial number is 257998 and it's a 6 shot 38 special it would be a 4th Change M&P and date to around 1915/16.
If it is R257998 and it's 38 Spl. then it should be a model 60 from 1979/80. And it would be stainless steel, not nickel.
Is it a 5 or 6 shot cylinder?
