This is a Adams Patent Small Arms Company breechloading revolver made around 1868. It is chambered for the .450 Boxer cartridge and has a capacity of 6 shots. The right side of the frame is marked ADAMS PATENT No. 866 along with the John Adams AJ trademark just above the grip. The barrel is marked "ADAMS PATENT SMALL ARMS COMPANY, 391 STRAND, LONDON”.
This exact revolver is pictured on pages 123 and 124 of the book “The English Revolver” by George Prescott.
This revolver was built to the John Adams Patent No. 1961 of October 22, 1867 which itself is an improvement of an earlier John Adams specification No. 1959 of 1866. Although introduced as the Adams's Patent Double-Action Central-Fire Breech-Loading Revolver, modern collectors simply classify it as the Model 1867. However, there is in fact two variations of this revolver. This one is referred to as a Model 1867A and the other as a Model 1867B. The two differ only in the method of holding and releasing the cartridge extractor rod. The 1867A uses a rotating lever whereas the 1867B uses a push button. Both designs were produced for the commercial market but were also were trialled by the British war department for use as a military revolver. As a result of these trials the Model 1867B was adopted by the War Department on the 22nd February 1872 as the “Pistol, Adams' Central Fire, B.L. (Mark II)”. This wasn’t to last long however as a few months later in August 1872 an identical model with an improved extractor design was officially adopted as the "Pistol, Adams' Central Fire, B.L. (Mark III)". The commercial version of this revolver is classified by modern collectors as the Adams Model 1872. They were used widely throughout the British empire.
Although this a commercial revolver it has an interesting British military inspection mark on both the cylinder and the left frame. The Crown/GS/M mark was first discussed in the 1976 published book "Adams Revolvers" by Chamberlain & Taylerson. The authors didn't know much about it then and even today its not really understood. The current thinking is that the GS represents the initials of George Stainton who was foreman of the Enfield Small Arms Manufactory in the 1860's and who oversaw the inspection of numerous arms for both the British war department and for colonial governments. The M may stand for "Manufactory".