One correlation that's been made is year of mfr, Stimpy point that out in a thread last year. For example, I've tried several 2005 barrel and frame combos that most fit fine, but not all of em did, but they were still very close, more than likely all they'd need was to clean up the chamber face which I've done a lot, a lot have excess headspace which makes it easy. If the 450M barrel has an ejector, you'll know it's an older barrel, at last report, the accessory barrels still were. Regardless, having multiple frames is an easy way to insure a second hand barrel will find a good mate.

Tim