If the gun is "new" and properly timed, a turn line has nothing to do with whether it has been fired. It does indicate it has been (mis)handled. By (mis)handled I mean the cylinder has been opened and then closed in a manner that I consider careless, perhaps because no one was considering it highly collectible and a concern or perhaps they just didn't know. Was it (mis)handled at the factory during manufacture, inspection or test firing? Was it (mis)handled by a previous owner? Who knows?
Firearms should be used, albeit sparingly for highly collectible pieces, IMO.
Most Diamondbacks aren't really scarce or rare, although they are no longer manufactured, which adds to the collectible factor.