Can't find this serial number on the Schwinn company page. The other photo clearly looks like the auto cycle embossed chain guard. Please note there are marks on the frame where the crossbar used to be.
That frame with no cantilevers is a Flying Star. That's the only frame of that style without the cantilevers. Most Schwinn middleweight frames had cantilevers. They weren't cut off as the seat stays look factory where they end.
The 58 catalog isn't all there, but here's a 56 illustration:
If you examine the photos closely you will see the marks on the frame where the cantilever bar used to be. One of the photos has my gloved finger pointing to the mark on the frame. The other mark should be just below the seat post if you look closely. I should have pointed that out.
Look closely where my gloved finger is pointing on the left. This is on an inverted frame. You can see the mark from the first bar.
Next, look closely at the photo on the right. You will see an outer circular mark where the weld broke. You will see this around a hole in the frame. Not sure why the hole is there. Just use it to locate the mark.
Keep in mind that both of these marks on the frame are on an inverted bicycle. I'm pretty sure this bike is the model I stated. I just wasn't sure about the date. I tried looking it up on a Schwinn serial number chart that I ran across and couldn't find anything with a number starting before the letter. Everything I saw on this other chart started with a letter and then had a number after that. That's what through me off and why I came here.