I have a little question : what is the wiggly line inside the board edge ?
It's to guide the attachment of the matrix to the board. The only way to identify pin 1 from pin 17 is to notice the key grooves on the matrix part. The actual matrix outline in yellow is showing the key grooves; the white wiggly line is for the silkscreen to help show this relationship.
In addition, if someone wants to use several boards+display side to side, maybe we should add a row of headers, with the DIM / SCK / SCI / VCC / GND shared with the already existing 6 pins header going to the arduino, and taking the SERIAL OUT of the 4th 74HC595 and wiring it to the SER pin of the "outgoing" header".
I was also thinking the same thing, but I wasn't sure of the best approach. Ideally, a ribbon cable with identical wiring to all boards would be nice, but I think you're right, two headers, one for input, one for output, may be required.
I don't think one Arduino could manage all the data required for two matrices, anyway. The generic 6-pin header is to go to any interested hardware, whether it's an ATmega168 or something larger.
However, I'm trying to make the empty space in the middle large enough I can optionally attach an Arduino Pro Mini to it. It would fit between the matrix board and the matrix itself, listen to commands in some way, and manage the refreshes. Any ideas there would be welcome.
Unfortunately, a Mini Pro + Matrix Board will probably be the same cost (or worse) than the existing Matrix Backpack at Sparkfun.