on
http://www.msxcafe.com/
in the forum
....................Materiel
.................................Upgrade CX5MII en MSX2
you find schematics and photographs for transform Mapper 64k to 4Mo (on board YIS604 CX7)
look the links for schematics : cx5m2mapper64k.zip cx5m2mapper4megas.zip in post #13
is a french forum but i can translate the technical part