I agree with 357 Python that it appears to be a Model 10-5 whose tapered barrel has been replaced with a heavy barrel from a 10-6 (or 10-4, or 10-3, or 10-1, or perhaps an early 10-8). Additionally, if the grip frame hasn't been altered to be a round-butt, I'd say it originally had a 2" barrel.
I don't know how I'd tell if the frame was altered or not. It'd be neat to go to a 4" tapered barrel to match the frame, but everything I've read would indicate that is not a cheap thing to have done properly.
(Sort of like the 19" barreled 1898 Krag that I bought at a LGS for $160.)