The 13K serial number prefix would suggest a 1977 manufacture. I would not expect to find a "bull" or heavy barrel on a 15-3 but I do have a factory 4" heavy barrel on a model 15-5. It was made in 1986 during the (1982 to 1991) timeframe when the model 14 was discontinued and the model 15 had to fill the void.

I certainly can't explain the stainless appearance . . . unless it was hard chromed. It's interesting though that whoever did the work left the hammer & trigger case colored and the extractor blue . . . as you would expect to find in a nickel finish.

Please post pictures if you can and perhaps folks here can see something that will help.

Russ