The first successfull slide action shotgun was produced by Spencer in 1880. The gun was designed by Christopher M Spencer and Sylvester Roper and was patented on April 21,1885. Patent # 316,401 if you are interested.
Spencer apparently produced guns in the late 1880's.
The gun was unusual in that it loaded through a gate on the sideplate and ejected through the top similar to Spencer's Lever action SG. Gun also had a lever in front of the trigger to cock the hammer.
Source: "The American Shotgun" (1973) by David F Butler, sold by Lyman Publications.