This Colt Huntsman was produced in 1976 and is what is known as the "Double S" variant. The zero prefix numbers created a problem when the serial numbers reached 099999S and rolled over to 100000S because serial numbers in that range had already been used in 1951-52 and began repeating. When the error was discovered, the 1336 guns that were already numbered received a hand stamped S prefix as seen on the pistol above.
The huntsman would be built for 2 more years following this particular pistol and would start production back up in the three hundred thousand range. More information on the huntsman ship dates can be found here.