First, welcome to the forum. If you have been talking to Painter777, then you are going to REALLY need some help!!!! Just kidding, if you have been talking to Charlie, you are well on the way to getting a good start in Carbines. In fact, most the people on this board are very knowledgeable and very helpful! Charlie is the best!

Looking at your questions, I am not sure how much accurate info anyone can give you at this point. Of the 10 different manufacturers of the carbine, 6 of them had numbers with serials starting as 4,xxx,xxx including Underwood. (Underwood also manufactured their assigned blocks out of order.) Underwood also manufactured their own barrels (plus contributed to the Free Barrel Program) so it is very doubtful that an Inland barrel would be original to an Underwood receiver.

Chances are that what you found in the gun shop is a re-built mixmaster carbine. And that is ok. Rarely does anyone find a "super collectable" carbine their first purchase. Just look over the carbine closely and make your purchase decision based on the weapon's overall condition. Shooters are good to have because you will want to SHOOT at least one carbine in your (soon to be built) carbine collection. plus it gives you a chance to get "up close and person" with a carbine you can learn about.

Good luck and good hunting and remember, carbines get hungry and they like to be fed LOTS of ammuntion!!!!