US Corps of Engineers (U.S.C.E.) strange cover ?

[Vertol]

Good afternoon,

 

I've found this item that should belong to the Corps of Engineers according to the marking but I am unable to find what it is for. I guess it's a pad or a cover of something (a rounded box ?).

It has a diameter of ~7'' or ~17.5cm, and a thickness of 1'' or ~2.5cm, it is in brown leather with an insert of wood, and has a small strap on one extremity and a rivet/button on the other (the leather is darker because still wet as I've put dubbin).

 

I thank you in advance.

 

Best regards,