This is a double action derringer with a concealed hammer which is primarily within the grip frame. The lever behind the ring is lifted to return the ring without firing and pushed down to release the barrel group to allow loading through a port in the breech of the frame. The grips are made of hard rubber ( Gutta percha ) and ivory being the only known other original. Blue or Silver finish (not nickel) or combination of both. Blue with silver frame or blue being most common and silver barrels more scarce. Screws can enter from either the left or right, mainspring set screw, none, or hole without threads present, throughout the serial number distribution with no apparent continuity. Serial number is on the frame under left grip.
Extensive study has identified approximately 140 known examples, being that this model has a quite delicate mechanism and was replaced by the Remington-Elliot Derringer "New Repeating Pistol" even before all were assembled, the survival rate of this model is expected to be quite low.
I've researched these firearms for over 20 years and I often hear screws enter from the right (or left) and that's more scarce. I can say quite difinitively it's completely random, there is no predominace of screws entering from the right or left it's just whatever or whoever felt like threading and assembling that day.
Also with the screw in the back of the grip, it's not a modification or variation at some point, it's just the variance of making the spring, if it needed it, it was added. I've even seen a frame where the hole was drilled and never taped. The serial range for ones with and without is just scattered. There were many things that they weren't good at and didn't understand metalurgy well, springs certainly are one of them.