I first saw this Spitfire on GunBroker at too high a price; asked the seller several questions and his relies were not forthcoming. About 15 months later it was listed again by the same seller at a more reasonable price and I decided that if it proved to be a mistake, I could always part it out and get my money back, it was.
When I got it, it wouldn't correctly chamber a round, the last 1/8" didn't seat, close inspection I could see a tiny shoulder like reaming of the chamber had never been finished. Took it to a somewhat local Gunsmith (48 miles each way) and it turns out that whoever first reamed the barrel had ruined it, the portion that they had reamed was too big somewhat out of round.
Looked online for another barrel with no luck for months, when I discovered this old thread https://johnsonautomatics.com/messageboard/index.php?/topic/2133-value-of-carbine/ and contacted Tony to see if he still had the spare barrel he mentioned. He did and we made a deal.
Took the barrel and the gun back to the same Gunsmith and waited, he had a several month backup at the time. But now the rifle is back he did a great job and even found front and rear sights for the rifle which it didn't have when I purchased it off GB.
The Underwood receiver was made in '43 here it is on the top shown with my CMP Standard Products .30 cal
Thanks again, Tony if you see this.