Dick, I don't understand why you think the sight is much more recent than the rifle.  On page 16-17 of Al's "Trade Edition", re shows this same sight in both the carbine and rifle versions.  It is described as the Model 1873.  The "V P and the Eagle" proof marks on the barrel, began at about 40000 per the book and the sight remained until the Model 1877 came along.  The range numbers on the M1873 were on the right side of the base (1 to 4 for the rifle and 1 to 5 for the carbine) and with the M1877 and later, the numbers were on the left side.  The slide on the rifle was graduated to 1100 yards (#11) and the carbine to 1200 yards (#12).  This sight retains those features without the correct screws as you point out. 

I have another carbine with the same M1873 carbine sight base, and slotless screws, but with a broken spring and leaf.  It is numbered to 5 on the right and except for the number of steps, is the same as this rifle sight. (photo).  That carbine is from a later period - probably arsenal altered - and has the low arch, more recent marks on the door, and the butt door. It does still have the oval cartouche and early band.
I agree with you that I could replace the sight, but with I will use a M73 Carbine - they're not super hard to find, right now although harder than the rifle version.  I also agree that these screws are wrong and I will correct those.  I'm guessing that this barrel was changed after the piece left the military and whoever changed it, probably was as unconcerned about slotted or slotless screws as they were about the proof marks on the later barrel. He probably used whatever he could find that "looked" right, and found a rifle version.  In any case, I'd prefer not to settle for a later sight.  I will certainly keep the rifle sight even if I replace it with a carbine.
Finding an early bar and ring will be tougher.  I may have to settle for the a later version for this part, but for the time being, I'd rather leave it as the owner left it.

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