Welcome to the forum, Les
Now, with such caliber, my previous humble perspective ought to be reviewed. Without a point of reference, only judging by the images, i thought this was not such a sturdy gun. This being a 'forty fiver', one may then think it was made for more argumentative purposes. Despite some (plausible ?) doubts that this type of multishot pistols was to mount on boat rails, it remains highly potential that its function would require it to be fixed onto some base or extension, and its trigger able to be pulled with a string or a wire. In line with that, i would reiterate my fantasy that the slight protuberance on the grip back works as a fixation spring. Can you confirm that ... or is it just an irregularity of the material ?
It is good to have you around; further riddles may be on their way to our appreciation