SLAM and Article 7

SLAM purchased the Ka Nefer Nefer mask in 1998, a full 14 years after the US became a state party to the 1970 UNESCO Convention. What about Article 7? It reads in part:  

The States Parties to this Convention undertake:

(a) [...]

(b) (i) to prohibit the import of cultural property stolen from a museum or a religious or secular public monument or similar institution in another State Party to this Convention after the entry into force of this Convention for the States concerned, provided that such property is documented as appertaining to the inventory of that institution;

That certainly would seem to apply in this case, Ka Nefer Nefer's mummy mask should be in Cairo Museum (or the Sakkara store). What - actually - have the US done to "prohibit import" of such items?