All of the single-action revolvers of the era (well, muskets, too) that I know about have a half-cock position. This may be what you are feeling.
There are two notches in the tumbler - one deep one that the sear cannot jump out of if the trigger is pulled - this is the first notch encountered while cocking and is the half-cock notch. The second notch is the "step" that the sear rests on until the trigger is pulled - this is the full cock position.
Half cock is used to allow the cylinder to spin freely so that you can load and cap the cylinder.
You will feel the hammer go "click" as it rolls over the half cock step. There should not be any resistance as this happens though.