I've come across a Smith and Wesson Highway Patrolman that somebody converted to .44 Special. I was trying to track down the age of the original weapon. It's a S&W Model 28 (I presume, since it's stamped "Highway Patrolman" on the barrel), serial number 175XXX.
Technically, it appears to be a "pre" Model 28, but just barely. That serial number should have an "S" ahead of the digits. Assuming that to be the case, and assuming there's no model number stamped on the part of the frame that's exposed when you swing the cylinder out, your gun dates from 1956 - stamping of model numbers was initiated in 1957. If you do find the model number there, then it's one of the first batch of Model 28s in the strict sense, dating from 1957. (With that S/N it could go either way).