If /IsaacKing/was-the-vela-incident-a-nuclear-bom resolves NO, this resolves N/A. Otherwise it resolves to all countries that knowingly and significantly contributed to the construction of the bomb.

The countries must have directly contributed to the test in some way: Providing transportation, collecting information about where to place the test, manufacturing parts of the bomb. Generally contributing to the nuclear weapons program of a contributing country is not sufficient.

https://en.wikipedia.org/wiki/Vela_incident#Possible_responsible_parties