I parked this overnight, thinking that all would be clear in the morning...
But, I still can't explain 13. I assume that there is a connection to the Cretan visitors responsible for a plain architectural style, and only have the second letter to provide (O?), but cannot find the word in Chambers. Maybe I've got something else wrong...
...and I don't see 39, so 2 unches missing there.
I've identified 20 cells comprising an old series, I assume from the shared cells that the remaining 8 cells to be highlighted comprise an inner 'circle' ?