I have a Defender as well, #L1562XXX that is also just marked "Winchester Defender 12 GA" etc. We both have low serial numbers as these things go, as some of the newer ones I've seen are in the 5 million range. My guess is that they didn't start putting the 1300 designation on them until later on. I think they also changed the design slightly, as the newer copy of the manual I have makes no mention of the little screw in the bottom of the bolt, but an older copy of the "Military" manual does. Perhaps they began putting "1300" on them when they modified the design. Then again, maybe I'm completely wrong.
In any case, they're good guns, and you got it for a very good price. Mine was also from a LE agency, but it's not marked, and it's probably been fired more than yours. But I like it, and I hear that the older ones are more reliable than the later versions, which makes it funny that they're usually cheaper, probably just because they're a little more beat up.
Anyway, enjoy your Defender, I know I do!