Hello mate and welcome to the forum, from the pics its hard to tell what colour it actually was, I presume it is a Heer buckle?
It is the Heer model indeed but found in a kriegsmarine coastal battery so it could very well be a kriegsmariene one.
I will ask him to make some better photos from the buckle.
Yes that would be good if you can, another theory if the grey is the actual colour of the paint and not part of the cleaning process maybe the original owner painted it ship grey? I have seen other buckles done this way.
He does not know where he has left the buckle....