Hi Jisx
a proof mark is marked on the barrel of the cannon/gun where the pressure from the explosion will be contained.
I would hazard a guess that the number represents the year the gun was in service and the oak leaves are just decoration, ,maybe associated with a manufacturer.
see this thread of one of my items showing proof marks on model cannon.
http://www.vikingsword.com/vb/showthread.php?t=24847
Regards
Ken