the 10-6 was introduced in 1962, and removed the trigger guard screw on the heavy bbl models, according to the standard catalog of S&W. There is a 4" 10-6 on Gunbroker right now with a buy it now north of $500.
I just bought a 10-7 2" in January for $190.00. I would say $300.00 is fair. but that's my opinion.
Great gun! A pleasure to shoot.