The radiator cap should be 13 psi. I prefer the lever-vent style. There isn't any thermostat.
Tell your parts house you have a 70-72 Datsun (Nissan) with coolant recovery system, and you need a 13 psi cap for it.
It seems that in the 70's Yanmar standardized on Datsun interchangeable parts for some of the maintenance items, I suppose to guarantee worldwide parts availability for years to come. How's that for over the top engineering!
CarQuest 33061 is what's on mine.
More Datsun parts matches, if this can help someone:
The 70-72 Datsun (Hitachi) alternator is a close match. It's the same basic model but lacks a rear dust shield and has the next larger diameter pulley. It's 35 amps, replacing Yanmar's 15 amps. A rebuilt one costs about $100 less. Close enough for me! Also that year Datsun voltage regulator is a perfect match - specs, mounting, connector and all.
Has anyone identified other Datsun part interchanges? Maybe some bulbs or something? It would be great to assemble a complete list.