Treshkin - these rounds are Greek, from the Spanish Civil War, and have been found in full boxes with an occasional standard Greek headstamp os the same year in the boxes. There is absolutely no question of the ENK identity. All characteristics of the rounds match standard ENK production for the same year perfectly, aside from the corroboration received by finding properly headstamped rounds in “I38A” boxes.
It may be that the “IA” letters have no meaning other than “in-house” identification for the use of the factory or other official sources. I simply don’t know. At any rate, if they have a specific meaning, I don’t know what it is.