The holdopen is the piece of steel inletted in the steel which spans the sides of the frame, it is pointed out in the photo below. In a Luger which has no holdopen the crosspiece is one solid piece with no machining.
The most obvious characteristics of a Navy Luger ae a 6" barrel and a special, two-position rear sight. It will also have Crown/M proof marks on the left side of the receiver, as shown below.
--Dwight