I believe that the adjustable sights only come on the 7.5 barreled models,(Redhawk frame??) whereas I have seen fixed sights on both barrel lengths on the newer, Colt style 'Old Army' .
I converted mine by removing the foresight ramp and replacing it with a half-round blade and removed the rear sight and replaced that with a simple notched block.
All carried out in my garage with simple hand tools,,,easy!!
Good luck,,,AE.