You might be right...

not totally clear in these shots I took of the specimen at Koblenz
http://www.armourworkshop.ca/gallery/image.php?album_id=51&image_id=2385&view=no_count

http://www.armourworkshop.ca/gallery/image.php?album_id=51&image_id=2387&view=no_count

more pics of that 231 here
http://www.armourworkshop.ca/gallery/album.php?album_id=51

http://www.armourworkshop.ca
here is a cross section from the Museum Ordnance Special #11 maybe they aren't supposed to be there as the rubber doughnut on the other part maybe all that is required.