The stock Datsun 1200 Rocker cover uses a push-and-twist oil cap. It is a classic all-chrome cap and seals with a simple round gasket. Some other Datsuns use a threaded cap that will not work in a 1200. By the way, this is where you add oil to the engine.
B10 Elephant Cap
B10 Datsun 1000 Vented Oil Cap
(Datsun 1200 caps are sealed)
Late Model Sunny Caps