What was on top of Diagram PA009B COVHOPs?

[F2Andy]

I am looking to make some of these. They are some great images here:

https://paulbartlett.zenfolio.com/procorcovhop

 

But none show the top. Would anyone have any idea what would be up there?

 

I am guessing a platform to walk on, accessed via the ladder, which is the bit the sticks up from the curve. Looks like three sets of three somethings on top in some images... Would there be three compartments internally? And I guess a hinged metal cover for the the openings. Would each compartment have three square openings or one longer one?