Nice. :35:Liberated from consignment at a LGS. S&W Model 10-6 with a Bucheimer holster. Dickered awhile and got my price. A heavy barrel, the lock up is tight and the bore looks good.
The Mod 10-6 was made in both 38 Special and .357 Magnum The original .357 Magnum versions were made for the NYSP and were the pre-cursor to what became the Model 13, which - like the Model 10 - was just another variation of the S&W M&P (Military & Police) line. The other fairly common but less seen M&P in the K frame was the Model 12, which had an alloy frame, and was the M&P Airweight model. There was another M&P Model, the Model 11, chambered in .38 S&W, but these were very scarce and rarely seen.
The D in the serial prefix was not unique to the M10-6 in .357 Magnum, it was used on all the K frame M&P models at the time, including the 38 Special versions. Your gun will (or at least should) have a D prefixed serial number on the butt of the gun, if you have over-sized grips covering up the butt you will need to take them off to see the S/N.
The M10-6 variation in .357 Magnum is sought after as a collectible, the overwhelming quantity of these were made for and delivered to the NYSP, but there are some that were over-run guns that were released into general distribution. The only way to know for sure if your specific gun was an NYSP version is to send for a factory Historical letter (cost $50) to determine. If it was an NYSP gun orginally it would have a premium on value, assuming of course the condition isn't terrible.
Do you have a .357 Magnum cartridge you could drop in the cylinder to check it?