Re: colt revolvers in 455
I think Marks I to IV are considered black powder only, with the Mk V a bit iffy.
The conversion of a .455 S&W or Colt to .45 Colt also requires trimming a bit off the rear of the cylinder or counterboring the chambers since the .45 Colt rim is thicker than the .455 rim. That means the .455 will have excessive headspace and possibly erratic ignition if fired in a gun converted to .45 Colt. (I suppose you could do a chamber counterbore just large enough that the ,45 Colt would sit deeper but the larger .455 rim would still sit higher, but the conversions I have seen had the rear of the cylinder trimmed down slightly.)