I think The ports in teh stern of the Cruizers where rather not used for chase ports as there would be no room to work a gun there and steer the ship at teh same time. I just can`t see anyone putting a gun where you need room to swing the tiller. i think they serverd more as ways to hand out cables in the rear for a tow maybe and or a kedg anchor cable.
I think that there are most probably lids in these ports tho i doubt they where hinged ports but rather plug type that would be taken out and stowed when not in use. I`d have to look up what perejus says in teh Brig irene book, i think he makes mention of this but i can`t remember for sure.
BUT your transom art is awesome