Sub Pens were in the World at War Expansion for the Original Version of A&A. I beleive they cost 6 IPC and were3 built in one turn. I found the final updated rules that applied;
6.11 Sub Pens:
a) Moving into/out of a sub pen: For movement purposes sub pens are considered a part of the sea zone they are built in, and do not require a movement point to enter, leave, etc. For all other purposes, sub pens are considered a separate sea zone/coastal area unto themselves.
b) A sub pen (and the subs within) can only be attacked by aircraft (follow 6.11 rules).
c) Any other friendly or enemy ships have no effect on attacking, defending, etc with regards to the sub pen.
Example: Germany has a sub pen with 2 subs in it on France, and a cruiser (all in the same sea zone). Britain decides to attack the sea zone with 2 battleships and a sub. Britain can only attack the cruiser since sub pens (and the subs within) cannot be attacked by ships. Germany defends with only the cruiser (the subs in the pen cannot assist).
d) If the land area a sub pen is adjacent to is conquered by enemy forces, the sub pen is immediately destroyed.
e) Sub pens fire each round, like an AA gun hitting on a 2 or less, against aircraft only. A Sub pen can hold up to 2 submarines.