Couple years ago I bought a 10-3 with a 4" heavy bbl well used. Tonight a friend brought over 4 guns and one is a mint 10-4 with a light 6" barrel. The latter is here because her husband is entering into Alzheimer's disease and they are moving. Aren't these 2 engineering changes only made for a couple of months? What are some of the rare versions of the popular model 10's? Larry