Gun values are based upon both scarcity and condition. Your Grandpa's revolver doesn't have much of either. You might find a roughly similar revolver at a gun show selling in the $250-$350 price range (assuming it is in original and shootable condition), so aside from its value to you due to its family heritage, it has only minimal monetary value to most potential buyers.

"Typically they are 38 special caliber so the 38 Long Colt could be worth more or less."
Regarding the two-caliber barrel stamping, there is nothing rare or unusual about it for guns of that time period. S&W M&Ps were so-stamped from 1902 until about 1907-08, up to around SN 100000 or slightly higher.