Brian, your grandfathers gun is from 1902 and is referred to as a 3 pin 1 screw lock. The barrels are Best English Stub Twist and at this time in 1902 it was available with 3 different types of barrels, Royal Steel-$20.00, Best English Stub Twist-$23.00 and Good Damascus $25.00. the F grade hammer gun was the lowest priced gun made, but at this time to get a higher grade it had to be special ordered.
Many of us still shoot these old hammer guns after we find that the barrels wall thickness is safe and the gun has been looked at by someone who knows these old guns. Good luck and if you decide to shoot it as there are a few manufactures of low pressure, low velocity shells out there like RST. Or if you reload or know someone that does, there is plenty of information.