NB: The U-20 requires a non-JEDEC 32-pin 128kbit x 8 EPROM.

In theory it's possible to use a more modern 27C010 JEDEC
pinout device but it would be necessary to use an adapter
board or socket to swap around pins 2 and 24.

The original EPROM part number is M5M27C100P and when using
a 32-pin device R24 (a 0 ohm jumper) needs to be in place.

Later v2.x and v3.x systems came with a 28-pin mask ROM and
need a jumper connected at R23 in place of R24.
