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ISO is the short name for the International Organization for Standardizationᵂ. The letters ISO were chosen as a language-neutral nickname for the organisation.
ISO is cast on various Datsun parts, like this choke cover
The ISO mark started appearing on Datsun parts around 1968 and became rare around 1974. For instance, the original Datsun B10 parts did not have ISO but some revisions started getting it.
The early B110 fuel pump was not listed as ISO, but the revision was listed as "ISO screw" in the factory parts catalog.
In the Datsun parts catalog, ISO seems to refer only to threaded parts. Datsun was fully committed to metric parts in the late 1960s, and at this same time they started touting their compliance with ISO. This does not mean that early B110 parts used Unified (non-metric) threads, only that Datsun was now advertising the ISO standardization being used.