Lister Petter AD1 no oil filler or dipstick

[The Genny Man]

Hi peoples, I have just bought a Lister petter Ad1 electric start and I was givern a dipstick, for the life of me I cannot find a hole to stick it in, also this engine has no oil filler with combined dipstick, see photo, the only place to fill with oil is the lole where the rubber bung goes, question is where do I fill it to, I assume till I see oil in the rubber bung hole.