With the Snakes, other than the BOA, you are dealing with production variations that make them less common and hence usually more valuable. But what is uncommon? What is rare? But, to name a few and all are factory original rather than something a gunsmith chopped up.... 3" Pythons, 2.5" Diamondback 22's, 6" nickel Diamondback 38spl, any nickel Diamondback 22, any BOA, to name a few. The basic models are generally fairly easy to find. For example, there are gobs of Pythons and Diamondbacks for sale. Anything that costs $3000 and up, I would want a factory letter to verify that it is the same as it came from the factory and nobody changed out a barrel to make it seem like a rare one. Some boxes for these guns might run as much as $1000, but you have to be careful there as there are lots of non-factory marked boxes floating around.