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 folding 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 to depths of up to 300m! 
 
Despite some apparent losses, mainly to the sides, the majority of the original textured grey outer finish remains intact.
 
In addition, this example is also an early production variant with the Kriegsmarine M/IV property markings, which have not been denazified or messed with in any way. Another KM ordnance stamp can be found to the top of the piece, located in between the open 'iron sights'. 
 
Whilst the lenses remain intact, the optics would require a full cleaning/service in order to return the piece to a usable state. However, the internal sun filter mechanism remains functional. 
 
Overall, a very interesting example of one of the more rarely encountered optics, which would make a great addition to any KM or U-Boat related collection!