[Inyo-Mono]

Before the invention of the tranfer bar safety system in revolvers first introduced by Iver Johnson in the 1890s (see picture) the hammer over an empty chamber or on the notches between chambers were the only safe way to carry a revolver. I can only speculate that with the dimise of the cavalry and the need to carry a fully loaded revolver on horseback with the hammer resting in a notch, that most sane folks carried with the hammer on an empty chamber.

When the hammer is on a notch the hand is not fully engaged and the cylinder can accidently spin into battery if the handler is careless. With the hammer over an empty chamber the cylinder is locked into place and cannot spin to a live round with being at least half cocked, much safer than notches on the cylinder. I do not know the added expense to a manufacturer of the tooling for notches cut into a cylinder.

[supercat]

When the hammer is on a notch the hand is not fully engaged and the cylinder can accidently spin into battery if the handler is careless.

I would expect that would be so on a typical revolver, although it would seem trivial to add the notches to lock the cylinder in place. Alternatively, on revolver designs were the cylinder face is flat without counter-sinks for the cartridge rims, I would think that it should be possible to design the hammer tip on a revolver so as to fit between two installed cartridges. To be sure, the cylinder could still rotate to an empty chamber, but that would seem pretty harmless.