Is it marked as a Spencer or as a Bannerman? Spencer production began in 1884 according to most references, ceased in 1889 when the company was sold to Bannerman. Bannerman continued in production til 1902 (or so: I've seen an old article from 1904 saying someone was assembling a few last guns from parts).
Remember the chamber was originally 2.5" (though some were lengthened), and the action was designed for that shell length. Black powder only. Every one I've seen had a damascus barrel, if you go to shoot it expect the usual lectures. Not SASS legal. The ones I've seen change hands lately were usually pretty beat up, and went for $200-300.
I've owned 2 (one shooter, one parts gun) and knew another guy who shot one pretty regular (when he could keep it working). They were a decent design, but most have seen hard use often with smokeless shells that stressed the works. My parts gun had excessive headspace, a chipped bolt face, and the magazine hanger had been brazed back on the barrel.
I always thought it was a nifty gun, but for a few years I was fascinated by the early repeater shotgun designs. At one point I had a Burgess, a Spencer, the last Bannerman version of the Spencer, and '93 and '97 Winchesters. Couldn't hit a thing with any of them, but that's just me.