See the few photos at the bottom of this page, around the bridge cannon: you can clearly make out the design formed around the cannon - we can assume that these relielf circles allowed the cannon's operators to lock their feet onto them (interpreted as dotted lines on my mod):
https://imagesdefense.gouv.fr/fr/cat...ype_filter=694
Here's the image I used from a u-boat model, where you can see this typical pattern around the cannon (of course, if you want to be exact, this dotted line should appear as a slight relielf and not as a hollow... :
