First, please use complete serial numbers only. Based on the partial serial number you mention, the date of manufacture was in the latter part of the year 1899.

The sight you have is a Winchester 3-leaf express sight, which was one of the optional rear sights for the Model 1894.

To loosen it up (and avoid possibly damaging it), I recommend using Kroil (penetrating oil) on it. Apply it to the pivot points and the pin that it pivots on.

The sight itself is designed for 50-yard, 100-yard, and 200-yard points of aim.

Bert