Alright, I got one.

I can confirm that the lower receiver is metal. Apparently, all the metal is aluminium and zinc alloy. Not sure what exactly the lower is made of, but it is cold to the touch and is heavier than the plastic TM lower, but not by THAT much. I might have to add some lead bits in case the Pulse Rifle is too front-heavy to stand on the mag bottom by itself, but I'll see once I finish the Pulse Rifle.
The Marui lower has an extra metal tube inside which is tapped in order to attach the front screw of the wooden stock. On the King Arms thompson, this piece is gone, since they tapped the hole in the receiver directly. It's metal, after all.
Marui mags fit just fine, even if you remove the innards of the mag (on the Marui Thompson, the "20 rounds" mag was rattling a bit after you removed the BB mechanism, but on the King arms, it sits firmly with or without the innards. It's great!).
The block of the tommin adapter sits a bit looser on the King Arms upper receiver, since the "nozzle" part of the upper receiver has a smaller diameter. But once you attach the rail of the tommin adapter, it should be just fine and the block should sit firmly in place. Otherwise, I'm simply going to pad the nozzle a bit so that it stays in place.
The upper receiver has nice real markings, none of that "Made by Tokyo Marui" stuff. That might be the reason why this version is hard to get in the US, since I believe there are pretty strict rules regarding real markings. It says:
AUTO-ORDNANCE CORPORATION
WORCESTER MASSACHUSETTS U.S.A.
and on the other side:
THOMPSON SUBMACHINE GUN
CALIBER.45 M1A1
NO. 280856
I believe the serial number is the same on each gun.
Disproven rumor: My Thompson had a little piece of paper attached to it saying that it was serial number 631. So the real wood and metal lower Thompson is NOT limited to 300 pieces worldwide. Some airsoft sites stated this.
The TM parts and King Arms parts should be pretty much interchangeable, although the King Arms outer barrel will not fit the Marui upper without some filing. And the real wood pistol grip of the King Arms does not fit the Marui, I think you'd have to file the wood down a bit.
The black of the King Arms lower receiver is ever so slightly different from the color of the King Arms upper receiver, but it's hardly noticeable.
What can I say, I just love it!

Now all I need are the aluminum shrouds from you, Matt, and I can finally finish my all-metal Pulse Rifle! Yay!
Cheers!
Roman