"...received a rifle..." Hi. You buy it or did somebody give it to you?
"...STD. PRO..." That'd be Standard Products of Port Clinton, Ohio. One of the nine major manufacturers. They made automobile trim accessories before the war. Also the second fewest made. 247,160 carbines were made by Standard Products starting in April of 1943. Issue carbines were only made during W.W. II
Start here.
http://surplusrifle.com/m1carbine/index.asp
Then here for a brief history of the assorted manufacturers. (Has nothing to do with Bavaria. Carbines were issued to Bavarian police and game wardens after W.W. II. Don't worry about that.)
http://bavarianm1carbines.com/manufacturers.html
"...minus the bag on the back..." That's a magazine pouch. It doesn't actually belong on the stock. The mag pouches were made to go on a belt, but they just happen to fit on the stock too. Buggers the balance and the mags are awkward to get at.
It have the same rear sight as the one on Wiki? Or one of the adjustable variations on the Parts, Variations & Markings page? The former is an early sight. 99.99% of all carbines went through an arsenal rebuild after W.W. II where all of the assorted upgrades developed during the War. It's highly unusual to find one that didn't. Mind you, none of the manufacturers made all of the 60 some parts.
Carbines are great fun to shoot(most will shoot circles around a standard Ruger Mini-14 too). Little or no felt recoil. Ammo(110 grain, .308" bullet at about 1800 to 1900 fps) is readily available from all of the ammo makers. You're looking for .30 Carbine ammo. Starts at about $20 per 50.
Keep your nose away from your thumb though. You won't bleed, but you'll know you got thumped.