This was an interesting design by Italian inventor Catello Tribuzio from Turin. He was granted a patent for his palm-squeeze pocket pistol in 1890. There were a number of these palm-squeeze pistols (Gaulois, Protector, Merveilleux) with the Tribuzio maybe the rarest of the batch. It utilised a top loading magazine and the frame design was extremely flat, to better fit into the palm of the hand. The ring operated piston acted as the trigger, operated by the squeeze of the middle finger. It was an inside-primed cartridge and not very powerful, with limited stopping power. (Erlmeier, Brandt Ref. 107C).

Source: Winant, L. (1955), Firearms Curiosa, Bonanza Books p84-86