OK. At least part of this seems to be specific to one revolver. Usually, the rear sight is slightly right of center to put the POI in the center of the target for me. On this one, a previously-unfired S&W Model 15-4, the rear sight is pegged to the far right, and the POI is still slightly left.
I don't know what might cause this situation, unless there's a problem with the installation of one of the sights, or if the barrel isn't screwed on straight. None of these situations is apparent to me.
It's going to a gunsmith this weekend for smoothing of the action; maybe I'll get some ideas there.
Guess it's time to move this thread to the gunsmithing forum.
Thanks again for the replies.