Battis, I looked at the picture of that 1903A1 sight assembly.  If you slip it onto your barrel and try to look through the hole in the base, doesn't a bit of the barrel get in the way?  If so, if you had the right sized pin (Numrich 517450?) and filed a shallow groove across the barrel, that would hold the sight on, with or with out the key.
Fact is, I wished I had bought the 03/03A1 assembly instead of the 1903A3 front sight.  It probably would've been easier for me to install.