Step 29 of the instructions tell you to assemble a fan for the turret roof. It lists no options or choices in this respect.
But in fact, there is an option here. The fan has a cover, comprising parts B42, B43 and S64. It's possible to omit that cover, and keep only parts B53 and B54.
The cover could be screwed down (it has wing nuts) to make a watertight seal. Obviously a ventilation fan will not function with such a cover applied. The cover was intended for use in submerged travel, which never happened in service as far as we know. Therefore the cover was therefore almost never used. It can be seen in a few photos, probably placed on the fan just to keep it out of the way. If combat were likely, the cover would be removed.