The Ruger No.1 rear folding factory sight utilizes a smallish beveled semi circular clamp and screw to secure said folding sight into the rib slot. Loosen the screw and remove. One can then tap out the sight. There is no lefty/righty to the sight base that I know of; but some things come out of habit. Take a looksee at Brownell's schematics for a 1A, the pictorial does not show the detail, but down in the SKU(parts listings 49,54,55)) you will see the folding sight, the Williams clamp, and screw.
If the sight is original, and been on a while, I might counsel a little Kroil or what have you, to loosen any oxidations, if any. Take note of the little bevel on the little beveled clamp, bottom clamp bevel to sight bevel; otherwise the clamp will be ineffective in holding.
