I don't know, thinks not originally, but you can try. A period correct cork gasket would probably rip by the protruding tab on the rod, but a rubber one might work and might prevent the constant rattle of the rod. The cap has very fine threads so there's little risk that the cap unscrews by vibration without a gasket.

A bit nuisance is that if the leather valve down the rod is worn and loose in the bore, oil will pool up on top of it so if you pull the rod up all the way, oil will gush out of the cap hole.