" Front sights an iron blade, with the rear sight a notch in the top strap just forward of the hammer. An extremely limited number of revolvers had adjustable (windage) rear sights, topstraps without grooves-and flat, and German silver blade front sight inserted in an iron base and secured by a screw; iron blades also known. Shipping ledgers note a few high front and "special" sights."
Quote is taken from The Book Of Colt Firearms but doesn't definitely answer your question. "Special" sights could be anything but my opinion is what you have is an after market modification.
They were chambered in 44 Russian, 44 German Government, 44 S&W and 44-40 so depending on your caliber you could have an antique on your hands. I have one that's circa 1883 but chambered for the 45 Colt...#@$*#@*!
Blair