And finally we are 100% done soldering, time to put the soldering iron down.

Shown below are the first two tiles in their final configuration while running off a 6V wall adapter. 

The first (or last) board in the series is fed power via an adapter cable. The first board in the series has an Arduino Nano mounted in headers. This Nano is responsible for sending the commands for the animation that is going to run on the whole display. The Nano that is permanently mounted on the first tile receives the commands from the master, updates its internal arrays with the values to display and drives the displays. Any commands that need to go to downstream boards are relayed.

Next step definitely is to order the mounting plate and 20mm spacers.