Descriptions by Lee Hart late Feb 2022, edited and discussed
with Herb Johnson. Last revision Apr 5 2022

Lee Hart: In my fashion, I didn't use any rare or expensive parts. I used a 27C16 EPROM as a hex-to-7seg decoder, and normal 7-seg displays (0.32 tall). The displays are scanned with a 74HC4017 counter, clocked at about 1 KHz. The 4017 sequentially selects the LED digits signaled by ROW1-6. [The ROM is addressed
by row and mode of operation to perform multiple functions. The clock also output-enables the ROM so
the display is only on when all inputs are active.] 

In LOAD mode, /WAIT=0 so the EPROM as addressed is programmed to only use the center-two two digits. They display the OUT4 byte as two hex digits. The ROW3 and ROW4 selects are routed to the EPROM so it knows which 4-bits of 8 to output as a digit. That's the only mode I "needed"... but I got ambitious.

In RUN mode, /WAIT=1 and the EPROM as addressed is programmed to use all 6 digits. The OUT4 byte 
00-7Fh is treated as ASCII ("text"), and displays alphanumerics (as best as a 7seg display can do it). 
Byte values 80-FFh controls 7 segments individually ("graphics").

RUN mode needs an interrupt handler to work. Each clock pulse to the 4017 generates an interrupt, via
Q6 (remove it if you don't have software to handle it). The interrupt handler then outputs the next character to OUT4, as it keeps track of the row count with a ROW pointer value. If ROW6 is active, it toggles EF2, to tell the interrupt handler it's on the last row, to reset the ROW pointer to start over.

The existing toggle switches and individual LEDs are still present, and work the same in LOAD mode as with previous Membership Cards. In RUN mode but without an interrupt handler, the 7seg displays will just display the OUT4 byte on all 6 digits. Without a monitor with program that uses the 6 digits to display the address and data, [a jumper P7 can ground the EPROM /WAIT input, so you could operate with two 7-seg
displays in the center positions]. - Lee