Well so much for that SP/ST serial number theory. Colt's strikes again.
You're sear block pin is too far rearward, besides being too big for an autosear pin. The dealer you spoke to didn't know what he was talking about. Pretty much what I expected.
The Blue Label Sporter models, Sporter Target and Sporter Match HBAR, were very mixed lot of rifles. Colt was transitioning from the old SP1 style lower receiver with it's large pivot pin, small fire control pins, no sear block and no mag fence to it's newer A2 type receiver with small pivot pin, large fire control pins, sear block and mag fence. For a while Colt's was using whatever they had on hand and you can find some real different configurations. This irritates the heck out of some people, but most of the Colt owner's find it "interesting". It also makes for very interesting collecting if you're into serious Colt collecting.