Lyman 48(M) Mauser Diopter Sight

The Lyman 48(M) was produced between 1911 and 1947. The "M" stamped at the side of the base indicates that it fits a Mauser reciever. As far as I know, it can be installed on both small and big ring recievers. Some say they also fit Husqvarna hunting rifles as well.

The Lyman 48 were the finest target sights made by the firm. They were factory installed on Carl Gustaf fm23 and fm23/36 match rifles from the 20s and 30s - so, this is an obviously an old peep sight design.