Here is the video of the second prototype in action:

The switches and buttons are marked and there is the meanings, left to right:

S: source address, enables addressed register onto the bus;

D: destination address, enables writing into addressed register fron bus or setting it to 1;

Write: performs writing from bus into register addressed by pair of switches marked as D;

Set: pushing this button makes register addressed by D set to 1;

R: resets all registers to 0.

Source and destination addresses are indicated by pairs of yellow leds, contents of registers are indicated by row of blue leds, registers numbered from top to bottom 0 through 3.