Hello, I have exact same fork caps on my 1976 Ossa MAR trials bike, they leaked all the time so I blocked them off .
Haven`t had any problems , but this is a trials bike , the forks would be working harder on a moto cross bike.
I tried to get some unvented caps to fit the forks but found they where differant {thread size or dimensions ? I cant remember}
I have found there can be differances between Ossa/Bultaco/Montessa Betor forks even when they are the same age, some
parts are unique to each manufacturer.
- dave