Initially, the B10 engines had rocker covers that had a stuck-on label that said Datsun 1000. Then sometime in late 1968 or in 1969 they started using rocker covers with the Datsun name that was stamped from the inside.

This seems to have remained through 1200 production untill the powers that be decided that the Nissan Name was to be used.
Tyrie has suggested that this took place with the introduction of the revised engine in '74 & I'm not about to argue with that.