The combo was a new purchse from Big 5 Sporting Goods, so it's not an older barrel. Since this is about the cheapest combo you can buy in a 500, I suppose they included a non-threaded barrel to keep the cost down.
I'm less disappointed after reading on the barrel that it is a modified choke, which will give me a lot of flexibility.
By the way, would you shoot slugs out of the modified choke 28" barrel?
I know I can shoot slugs out of the 18 1/2" barrel, but I think it would be harder to aim if used for hunting purposes, especially since it is only a single bead sight.