Extremely rare Luftwaffe ZG26 flight protection helmet. a so-called "Fliegerstahlhelm". 
Example with period time red color and a hand-painted unit emblem of Zerstörergeschwader 26 to the side. These helmets were painted in red color to help find and rescue crashed crew members. 

Attached M34 liner stamped size 69. The shell looks to be "ET66" marked but it is extremely hard to find. 1940 dated chin strap. The lining looks to be originally attached. The rivet prongs are flat. The rivets show an identical red color to the outside. There is some damage to the lining and the chin strap is not too soft anymore.

An extremely rare helmet issued in 1944/45. 

A nearly identical type is show on page 476/477 of Ludwig Baer's "Vom Stahlhelm zum Gefechtshelm. Band 1 (1915-1945).
Some rust.