From an image posted on msx.org foruns, 

the following pinout was obtained:

Button    Pin(s)
UP          8
DOWN        5
LEFT        6
RIGHT       7
A           2
B           1
SEL         8/5 (through D1)
RUN         7/6 (through D2)
SW          4
COMMON      3

and the schematic was drawn:

finis!