Srb & Stys 4x15 U-boat 2cm deck gun sight 'bmk'

A scarce original 4x15 optical sight for the 2cm U-boat deck gun, complete with the mounting base and fold-down original rubber eyepiece. 
 
Produced by Srb & Stys under the 'bmk' maker code, this optic is made from bronze and was designed to be both water and airtight - enabling it to be left fitted during any emergency dive with the submarine descending up to depths of around 300m! 
 
It still retains the majority of the original textured grey outer finish and the lenses remain intact. The optic is still usable, with internal crosshair aiming reticule, and the light filters are also fully functional. 
 
Avery interesting example of one of the more rarely encountered optics which would make a great addition to any KM or U-Boat related collection!