If you look closely at the lug there is a nice goober where something hit that hard. The break is at what would be the weakest place on the receiver, so if it was dropped and hit some hard object, that is a likely place for a break. At that point, the tube does nothing more than hold the spring in place.

Consider that there are some carbines, probably a lot of carbines that have unwanted holes in the walls of this hole where the drill wandered during manufacture. I believe that not all were considered scrap and were sent out as good receivers. It is possible to see the spring on some receivers when viewed from the top at the slide stop recess. I'd look for any cracks, and just use it. Maybe dress the inside edges with a small file or stone to eliminate any place for the spring to catch.