It seems that it shows markings of the hight of several parts of the boat.
Kiel = Keel
Bug = Bow
Heck = Stern
Brücke = Bridge (the lookout platform of the conning tower in this case IIRC)
My best guess is that this was used to go decks awash or just submerge certain parts of the sub...I could be wrong though.
However I don't get it that on the right it's one tube with two very different scales...