New England Custom Guns makes a ghost ring site. Available through Brownells.
P.S. Received the sight and it's a nice piece of work. Pretty expensive though. Haven't put it on the 550 FS yet, as the ten-day wait for the rifle is over next Monday.
P.P.S. The receiver sight required a taller front site. After measuring and eyeballing and fumbling with calipers I figured about 3mm higher would work. That is, coincidentally, the tallest that NECG offers. I ordered the "Sourdough," a patridge front with a shiny brass face. I prefer posts over dots. Took a lot of stoning to get it to fit, but it finally fits up snugly and it looks good with the rifle. This weekend I'll take it out and see if rear adjustments will cover the new front height.