I call them hybrids, rather than mix and match pistols. They're all over the Internet. Colts with Remington Rand and Ithaca slides. Remington Rands with Colt barrels (I have one, a '43 RIA rework). A lot of them are genuine arsenal reworks with the proper stampings to prove it. You'll see Ithacas with Colt frames and vice versa. And it's well documented that early Remington Rands used Colt safeties and slide stops left over from WWI. I think Colt furnished about 200,000 of them to RR.

The thing is, a lot of people don't seem to understand John Browning's basic concept for the 1911. Without going into a whole lot of detail, the pistol was designed and manufactured with the concept of parts interchangeability being a major factor in determining its suitability as a military pistol.

You might find a Remington Rand with a Flannery barrel. Flannery barrels, usually associated with WWII Ithacas, were a staple for field replacement in all brands of 1911A1s.

Some G.I. around Bastogne finds a 1911 in the snow. He decides to replace the High Standard barrel in his Remington Rand pistol with the High Standard barrel from the found pistol. Since HS barrels were standard in the Remington Rands, who will ever know the replacement barrel isn't original to the pistol? Answer: No one. These parts were not numbered to the gun, period. The pistol could turn up in 2016 advertised as "original". Is it? No. But who cares?

I'd be surprised if I ever find a WWII .45 that doesn't rattle a bit. It's just totally irrelevant. These aren't target pistols.

Without serious documentation, the question of "originality" will always exist with a lot of these old pistols. It's a moot point far as I'm concerned. A random arsenal rework is no less historically significant than any random 1911A1 that appears to be original.