PET 2001 6550 RAM R/W?

Hi all,
I'm going to repair a PET 2001, the board with 6540 ROMs and 6550 RAMs.
I'm still in the phase of studying the schematics to understand this
particular design.
Now, I'm not really getting why the R/W signal to all the RAMs,
including the two video RAMs is ANDed with the /SELE signal, can
anyone shed some light here?
On the 2001N board, the RAM R/W signal is just the CPU's signal plus
one buffer  and two inversions.
I'm a bit puzzled.

Thank in advance.
Frank IZ8DWF

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