And for reference, the card pinout on
http://www.vintagebox.de/elek/elekd50card.html#p2 is correct, even if the physical dimensions they mention are bogus

Let me copy that here:
1 | 2..19 | 20 | 21 | 22 | 23 | 24..31 | 32* | 33 | 34 |
+5V | A0..A17 | MR | WE# | CE | OE# | D0..D7 | GND | VBAT | PD |
*Pin 32 (GND) is the slightly longer one.
A0..A17 is the address bus (card input only)
D0..D7 is the data bus (bidirectional)
WE#, CE and OE# are standard SRAM Write Enable, Chip Enable and Output Enable signals (WE# and OE# are active-low, CE is active-high).
MR is the write protect switch - connect to 5V for write OK, to GND for write protect. This is just an indication to the synth, it can still overwrite the card if it decides to - the write protection is implemented purely in software.
On 4M PCM cards, this pin is repurposed for A18.
VBAT carries the battery voltage (nominal 3V for a good battery). I connected it to the 3.3V rail via a resistor to keep the synth from whining about low battery. Do add the resistor - the Akai MX1000 likes to pull a lot of current from this if I connect it directly to 3.3V, I suspect a clamping diode in the Akai.
PD is Presence Detect, connect to +5V for the synth to detect a plugged card.