AFAIK the SP guns were/are Wingmasters that were labeled differently, with a phosphate/parkerized finish. I stand subject to correction there, i don't own any SPs and never have.
There's one Express gun here with dull-finished walnut furniture- will have to call Remington with the serial # to check the date of mfg on that one, but it was early into the household among the clutch of 870s that live here. It is the only Express I have ever seen with walnut furniture, but it has dead straight grain- no fancies at all, a flat finish, and the distinctive impressed checkering of Express furniture that is commonly beech or birch. There was a short-lived 870 called the Sportsman model that was transitional to the Express line as far as I know, I have run across a couple of those here and there. But they were a bit nicer than Expresses in finish and thus priced higher.
SPs are also regularly priced higher when I have run across them in my travels, more akin to used WMs in price. Since I am admittedly bottomfeeding in my search for another Cinderella gun to rescue, I don't often spend a lot of time examining the spendier used 870s that inhabit the racks.
According to Kuhnhausen, both the SP guns and the Express guns were both introduced in 1989 FWIW.
lpl/nc
Update: Called the nice folks over on t'other side of the state. The Express I have with walnut furniture was made in 1994, per the serial #. Not as old as I had thought, and of course I cannot say for certain if the furniture on it now was original to the gun, since I didn't buy it new.
According to the Nice Remington Man I talked to, the SP guns are finished inside essentially to the level of Wingmasters but have a "black oxide finish" (his words) where Wingmasters have a polished blue finish.