Why do the end letters vary on some G-Shocks model numbers?
Some G-Shocks that are the exact same watch will have slight differences in the letters tagged on the end. Some times this is just a mistake by a retailer, but it's often the result of Casio varying the suffix of model numbers for various markets.
G-7800B-1 = abbreviated model number (usually export)
G-7800B-1DR = export model full number
G-7800B-1JF = Japanese domestic model number
As long as everything is the same up to the number after the second dash, it should be the same watch. There might be a different box or packaging though.