I have a Williams "recerver"sight on my 500 magnum.  I cannot tell you the model, but it is not the type NEF supplies with the cowboy style 45-70, but rather the larger one.  I have target knobs on mine.  This particular one extends out past the breech end of the barrel about 1/2 inch when installed.  Since my 500 is an extractor (as all 500's would be) the sight is in the way a bit when loading and particularly unloading.  Not a big deal, but it is the case nonetheless.  Next time I will go with the small sight which NEF uses on the cowboy rifles.  I kept the standard firesight on the front that came with the rifle as it works just fine.  The standard rear sight was easy to remove, just two screws.  I then used the two tap screws I removed to mount the Williams sight and put them in the empty holes left by the standard sight.  There is a bit of epoxy or some material on the barrel that was under the original sight, but I have left it in place rather than scrape it off.