There is more reason to consider spending the money to letter a Colt single action than a Colt percussion revolver.
Colt SAAs often have switched barrels, changed calibers, different grips. So your 1888 Colt single action in .45 and with 4-3/4 inch barrel might have left the factory as a .32-20 caliber SAA with 7-12 barrel. A Colt Navy will be in .36 caliber and with 7-1/2 barrel only, and so you won't need a factory letter to confirm that.