I just picked up a 642 "Pro" last week, and I have to say that there's something special about this revolver...I can't quite nail it down, but I've been noticing some little things that have been done 'right' here.
I had a standard 642 that I purchased almost two years ago, and there wasn't much special about the gun. I traded that revolver not too long ago, and I can't say that I've missed it.
But I am a 'revolver guy' at heart, and so when I had the chance to order one of the "pro" 642's, I did, and I'm really quite pleased with this gun.
The rear of the barrel is cut cleanly, with a B/C gap of .007" measured from both sides. I notice a slight bevel on the inside of the frame window (just to the rear of the model and S/N), and the engraving/etching (or whatever it's called) of the S-W logo, Airweight designation, etc., is sharp and precise...more so than any other S&W revolver I've owned.
After running 50 rounds of lightly-loaded DEWC's, mixed with some Remington 158gr SWCHP +P's, I find it extremely easy to clean the barrel and chambers, and there's very little powder residue/blast crud under the top strap at the rear of the barrel.
The fact that this revolver has no internal lock, and also accepts moon clips is just 'icing on the cake'...when I ordered it I thought that those two features were the only non-standard items of the "Pro" series 642...now I'm not so sure.
This revolver appears to be exceptionally well-made, and I'm really happy with it. The action feels solid and the trigger is smooth.
The moon clips (3 of them) that came with my 642 measure .025" in thickness, and R-P and Speer brass loaded in the moon clips functions without any binding. For me, the biggest advantage to the moon clips is that extraction of all 5 empties is quick and precise, without any cases getting hung up with the short stroke of the ejector rod.
While I could use either speed strips or my HKS speedloaders for reloading the cylinder, I'd really like to find a good way to carry and practice using the moon clips, and then decide which method works best for me.
After my back surgery Friday, and when I'm back up to speed, I'm looking forward to trying out different loads in other brands of brass (Winchester, etc.), to see how they work in the moon clips. But even without the cylinder being cut for moon clips, this 642 (to me) is a very special revolver.