They may issue a new ser# w/an 'ATF' or 'BATF' prefix for application to the revolver. That's been the case in two instances I've seen.
One a Colt SAA first gen with matching numbers that was stolen, the numbers ground off except for the one on the trigger guard was not completely gone. Gun returned to original owner after court case. New 'ATF' ser# stamped into the side of the frame dispite knowing what the original was.
Not a nice clean job like the Bonnie Parker Colt either!

re: the 45Colt to 45AR brass thing... On some cartridge conversions of rimmed cases where the parent cases rim is thinner than the case being formed,,the parent case rim can (sometimes) be bent or rolled forward a bit to make up the extra few .000 thickness.
You need to make a die to swedge the case, base first, into it to fold the rim forward and there may not be enough rim on the 45Colt to do that. But it's a thought. It's pretty common when converting some brass like 45-70 to odd Euro thick rimmed metric calibers.
Personally I'd just use 45acp brass,,most any gunshop or Cabellas type place must have a bag of that around.