earliest Reilly S-L in my database - like Darryl's gun it's a pin fire and looks as if it could be on either side....no SN mentioned so it can't be precisely dated but probably mid-1860's.
I wonder who created the original patent for a side lever? I have this patent mentioned in the patents found on Reilly DB - was it Needham or is the Dougall Lockfast patent by definition side-lever?:
1860, May - J.D. Dougall “Lockfast” action patent 1128
1862 - Needham patent 1544, rotating single-bolt snap-action side-lever