Welcome 3ky,
With no signs of any import markings...you did very well for $700. I've been seeing imports fetching that much lately. With your serial number being in the 5,383 mil range, you've got a Inland we sometimes call a transitional. Many changes were being implemented near this range. The round bolt and rear adj sight could very well be original or correct for it.
Is the oiler slot on your stock oval shaped or a I cut shape?
That stock would normally be found on a National Postal Meter. Your Inland would be found in a type III lo-wood marked HI or OI in the slingwell. With the Inland sized Crossed Cannons on the right...near the oiler slot. Those early NPM stocks are getting hard to find. A good starter book for you, if you don't already have it is U.S. M1icon Carbines, Wartime Production by Craig Riesch. It has some errors but is worth the under $20 price tag.
To mount that 30rd mag you need a type IV mag catch. They are fairly cheap to pick up. It has the support arm on the left to help carry the weight of a fully loaded 30rd mag.
That sling will sell between $100-$150 about any day.
Now sit down, look at all the markings you can find and take a deep breath....it's just the start !!

Cheers
Charlie-painter777