Pardon me again, everyone---I think I might have answered my own question as to the SE-19's vs. the SEMC-28's.
Found this on ebay (the auction has ended, so you'll have to scroll down to view the pic of this guitar): http://cgi.ebay.com/Sigma-SE-19-Acoustic-Electric-Made-JAPAN-w-extras-/220644619262?cmd=ViewItem&pt=Guitar

This guitar is identical to my SE-19---the inside is also stamped with "SE-19". I personally feel the guy sold it for waaaaay too low (perhaps his frustration with the pick-up had something to do with it---I had the Martin factory in PA replace mine in 1992, and I've never had a problem with it since). Also, the inside of the guitar looks pretty dirty, so I'm guessing it either had no case or a cheap gig bag (he states he bought it originally off eBay also, so who knows where all this guitar's been).

Thanks everyone for your indulgence---didn't mean to leave so many posts, but I kept being riveted to more and more searches on Sigmas and finding more info.