I've been looking at S&W model 19s on gunbroker and some are described as "pinned and recessed"; I see the pin above the barrel in the frame. What does the recessed mean? Also, when did S&W do this as it seems to describe older guns. Furthermore (sorry) are these guns as strong as the newer guns? Is there a structural difference between models 19, 27, and 686? I know some of this could be answered by a good gun reference book, but I don't possess one right now. Thanks a bunch.