The overall design brief for this project is as follows:
The initial prototype was a proof-of-concept to show that it could be done.
The four LEDs were red, green, yellow and a green-yellow (because a blue LED was deemed an unnecessary luxury for a proof-of-concept), and it used an FTDI USB to RS232 converter.
The second prototype was an exercise in reducing the size.
The green-yellow LED was replaced by a blue LED, because the visual impact was well worth the increase in cost for the colour, and it now used a single IC - a Microchip PIC with a built-in USB transceiver.
It was registered as a HID device - which made a cross-platform API very easy to implement, and no drivers were needed for any operating system.
The third prototype was smaller stlll, and used the PCB itself as part of the USB connector - much in the same way that some very small flash memory devices use.
After re-laying out the circuit, it was made significantly smaller - although it was getting to the point where making the PCBs in-house was complicated.
Six of these were made for the client to distribute to interested parties.