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...On reflection, this is probably one of the rarest original MK14 documents around, so I'll scan it and post it here - my scanner is so old that it only works with Win XP, so I'll have to drag out another semi antique in order to do that..
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I'd certainly like to see that when you get the time!
I think you are right that this difference is to prevent the PROM from being enabled every time A11 goes low, possibly because it might interfere with the VDU board, and they used the "read or write" line because it was there and the inverted RDS wasn't! I also noticed that the two spare AND gates are marked "These gates are used on rev 5 boards" so maybe the schematic is of rev 4 and the 2 gates were used to more completely decode the PROM address as you described. Its odd they didn't show them on the schematic when they added this remark but it was Science of Cambridge after all
I think that clears up the mystery for me at the moment but as I said I'd love to see that letter when you have time.
Ian aka Slothie