Just picked up my little 32/20 WCF after some work at the gunsmith to supply and fit a new firing pin, fit new rear sight, check headspace and general checkover. It is a fairly old one, 1885 Model low wall single shot made 1919. 

It was given to me for parts as it had no firing pin and the timber and woodwork and bore were pretty rough, I have other Winchesters in the same model and the owner couldn't be bothered with the hassle or cost of importing a rifle part for "an old clunker that was unshootable anyway." 

Couldn't wait to try it out so I set up a 10m air pistol target at 50 yards and to my suprise all ten were well on the paper in an even 4.5" group. Very pleased with this as the sitting position with open sights in a 25 knot screaming SW'er is not conducive to good grouping. 

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BSA smallbore rifle sight from the same era fits perfectly on the Winchester tang and folds down to allow use of the standard open sight 

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5 shot group at 50 yards of 72mm

GF,
From the looks of it, that seems to be a BSA model 8 tang sight, as made for the small-action Martini rifles (cadet-size).  These are quite serviceable and accurate. FWIW, I have one with a conversion plate, to fit on a 'normal' tang - these have the usual Martini screw lay-out (L and R, instead of fore and aft).