I too have a back action hammer sidelock bearing the W Richards name. It also has a left side lever opener & carries Birmingham proof marks which date it as a Pre-1887 gun. It very much resembles the gun in the picture, but of course, there were others which also looked similar. Several of the Higher Echelon British made side lever guns though had right side levers. They seem to be more common than left side levers.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra