Beachbumbbob:  Unless someone has aggressively filed or machined a 1901 Rifle sight slide to fit the 1901 Carbine sight ladder, your slide is a Carbine slide that wear has probably defaced so the small 'c' can no longer be seen.  The Carbine slide width is .235".  The Rifle slide width is .289".  A difference of .054" (54 thousandths).  The Rifle ladder corrects for drift and the Carbine ladder does not.  Attached is a photo contrasting the Rifle and Carbine 1901 rear sights (the Carbine sight is the upper one).  Notice how fragile the "c" mark is on the slide.

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