This might shed some light:
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Posted Sat May 27, 2017 1:12 PM
The 7 or 8 bit refresh has to do with the memory chips.
Older DRAM's use 7 bit refresh.
The newer (and faster) ones had 8 bit refresh.
So some memory upgrades to 256K needed an extra bit for the refresh.
That was created using a counter.
Have a look at the attached schematics. (a rework from the original design)
So I assume that Atari was planning to use different memory chips and they would need another way of multiplexing.
Hence, two different Fraddy-chips.
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