Para nailed it!  The 1901 sight has the aperture in the plate of slide.  The 1902 sight has a flip-up aperture, know as the "Sgt. Peep."  This plate has a pronounced half circle at the top with a 0.04 dia. aperture in it.  When you drill it out to 0.07 dia. (as large as you can go without destroying the plate, you'll wind up with a hair thin ring on top.  I built the top of my plate up with JB Weld so when I enlarged it further to 0.1 dia., I still had a defined ring on top.  Hey... it works for me! 

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