Your Model 87 Winder Musket was manufactured in the latter part of the year 1918. The Lyman No. 53 receiver peep sight has the patent date "JULY 16 '07" marked on it.
Based on what I can see of it in the pictures you posted, it is in poor condition. The stocks appear to be refinished with varnish (they were originally oil finished), the receiver frame has considerable bluing loss on the one side shown, and one of the (4) 6-48 screws is missing from the sight.
Better quality pictures might convince me that the condition is better than it appears, but at this point, it is worth no more that $600.
I wrote an article about the Winchestet Single Shot Winder Muskets a few years that was published in the "Winchester Collector" magazine - http://www.winchestercollector.org/magazine.shtml