Fields of Despair - GMT P500.
Rule for reads:
Naval Blockade/U-boat Offensive: Add one EP to the Naval
Warfare draw bag. This may be done as secretly as possible.
No player is ever allowed to look inside the bag. (Limit: CP 2,
* Note (Optional Rule): The Allied limit is reduced to one if Great
Britain is neutral (13.1).
Should it read "Add one cube?" If yes then AP can add 3 blue cubes and the CP 2?
EPs are defined as cubes (2.7). You have the procedure correct.