That is a special order delux 1873 with many desireable features, manufactured in 1884. The "A" is not actually part of the serial number. Take several detailed pics in better light. Skip The other sites and go straight to the Winchester Arms Collector Association forum. Bert and twobit are there most every day.
http://winchestercollector.org/forum/winchester-rifles/
As Scorch said, definitely start the ball rolling on the Cody letter. It will tell you the exact configuration of the rifle as it left the factory. $70 cost, but WELL worth it on a rifle like that.