just found this on eBay and I can confirm it is in fact a 3.9! Not one of mine
https://www.ebay.co.uk/itm/Datsun-2...-/154309713901?_trksid=p2349624.m46890.l49286
https://www.ebay.co.uk/itm/Datsun-2...-/154309713901?_trksid=p2349624.m46890.l49286