I am not an expert, but I can give some advice. The allen head screw attachment and the grooved trigger are indications of this being an early production gun. Original front sight would be a medium size brass bead, not sure of the exact size. Yours is no doubt an improvement. Trap and skeet guns would have Bradley red or white bead front sights.
SX1s were introduced in 1973/4 and were produced until the 1984 or so. They are very well built, heavy, and were quite expensive, more than a Browning A5, so they did not sell that well. The allen screws holding the rib can come loose, and if not tightened will eventually shear from recoil. Nothing that a dab of Loctite won't take care of. The trigger assemblies with the grooved trigger shoes are adjustable for slack. The newer trigger assemblies supposedly have better trigger pulls, I have not noticed any meaningful difference between the two designs.
The gun has a few weaknesses. If you have not changed the recoil buffer that fits in the back of the receiver you need to do so, they tend to disentegrate after 30 - 40 years, and shooting the gun without the buffer will eventually damage the bolt and receiver. The magazine tubes can rust if not cleaned and oiled. Wright's is one of the best gunsmiths for SX1's, and they keep parts like recoil buffers in stock. Nu Line also has parts and service for SX1's.
Most people that have these really like them, many consider them the best automatic shotguns ever built. You can find out a lot of information on Shotgun World in the I Love My Winchester forum. You can read posts by some REAL experts there.
http://www.shotgunworld.com/bbs/viewforum.php?f=60
