The Main Deck is where most of the changes have been made post fishing era. With exception to the furthest vent shaft (now used for the generator(s) exhaust pipework) all of the other vents, and stairs to the transmitter room, were installed as part of the Radio Caroline modifications.
As you can see from Slide 1 of 6, opposite, during the Freyr fishing days there were no vents or staircases - just hatches for lowering the catch down into the Hold.
The square deck protusions were used for raising the fish 'bins' off the deck so that cleaning water could pass underneath. For the Ross Revenge this was in the form of a raised deck but as you can see from the video for her sister ship, Mai GK346, the bins could be raised to a higher level by the use of pedestals.
Forepeak Deck | Stern Deck | Monkey Island