Daniel suggested This page which looks very similar to yours but not mechanically identical, and there are others like it on the net (as you note).
The rear view shows rows of many probably identical ICS and a few others near the input connectors. If you had one of these 'in your hand' trhe IC types would tell you quite a lot.
However, Daniels page has the line "Module interface: HUB40"
HUB40 is an LED panel driving system and Asian sourced interface cards are widely available. It is reasonably likely that your system is "HUB40 based". A look at the connectors on the rear of your system would go a long way towards confirming this.
Here is an example of a HUB40 transmit card - cost $5.
And a page full of them here - same supplier
etc, etc !!!
You belatedly mention HUB75. This is also an LED panel driving panel with many cards available
HUB75 TX pinout