Well from what I can see its possibly a well worn German leather greatcoat, a hard to find piece in good condition that invariably has some form of marking on the interior,,, by removing the the pebble buttons ,I,m afraid you have actually removed authenticity and devalued the coat. Personally I would re attach the original buttons using original German thread, these were worn normally by ranking officers, SS and even Gestapo, also Submarine officers, I ,m not 100% sure but the brass studs could be attachment points for a hood, could you check under the rear collar, see if there are any more