The firmware for the electronic stamp is downloadable here:
A movie of me using an earlier prototype (using another kind of cartridge) is viewable here.

The pinout of the cartridge seems to be mis-printed in the magazine (amongst others, R3 is missing) so here is the correct pinout of the model 40 HP-cartridge:
Pinout cartridge. Seen when looked at the back of the cartridge, with the printing head pointing downwards.
GndGnd   GndGnd
R4Gnd GndR1
R0Gnd GndR5
R3Gnd GndR2
C4R6 R7C
R0-7 en C1-4 are used in the schematic, the rest isn't.

© Jeroen Domburg