Great post deadin!
I will only add a couple of pics I was going to post anyway.
This of how far GI 45acp ammo drops into an un altered S&W 455 chambered cylinder.
..and a couple of a Smith 455 cylinder altered to take 45 (Long) Colt ammunition. Not a very well done job but it shows what is done in the process.
The other pic shows the fresh cut lengthened chamber w/original blued throat.
This cylinder has an 'in the white' extractor fitted. No ser#'s on either part.
Maybe some NOS parts never fitted that someone intended to make up a conversion with.