First of all, thank you all for watching and expressing your appreciation, for the feedback and Stephan for Blom's description/translation.
@Uwe
The gratings must have been removable. Either lifting the gratings out of a frame or out of notches, both must have been possible.
I am following largely the drawings of Otte Blom :
Blom does show gratings with a frame, but only on the lower gundeck :
Maybe it is a typical Dutch construction. Vasa (built by a Dutch shipwright) also shows this type of constuction:
And on the contemporary Hohenzollern model of a Dutch twodecker :
On these pictures you can also see that the Dutch at that time might have had a more simple method of constructing the gratings themselves unlike the English (also described as such by Witsen). The longitudal battens were simply fastened to the beams without or with only very shallow notches in the supporting beams.
Further detail drawings of Blom show gratings of the larger hatches even divided into 2 parts supported by a removable beam in the middle. I must say his drawings are not all consistent, some show 13 longitudinal battens, some 2x7 +the removable beam (makes 15) and an other (2nd in this post) even 16.
On top the gratings on the "verdek", below that the gratings on the "geschutsoverloop" the gundeck.
The model William Rex shows an other type of construction, but that is a late seventeenth century model, built about 35 years later than the 7 Provinciën was completed.
So, what is correct? As there are no contemporary shipwright's drawings of those Dutch ships we have only the guidelines by Witsen, these temporary models and the drawings/paintings by the marine artists like the Willem van de Veldes.