I was surprised and amused to take my Model 642 down when I first got it to find a single action sear. With the side plate off, one could manipulate the gun to operate with the single action sear. Gave a nice trigger pull too. Typical Smith & Wesson single action feel, like breaking a thin glass rod. One supposes that all J-Frame hammers are actually finished out the same prior to the steps taken to form unique DAO style for hammers installed in the Centennials.
Try cycling the action some time with the side plate off. The single action sear doesn't come into play with the double action operation of the lock work so its removal won't have any affect on smoothness.