The SPDIF DAC has been around since 2005
and there are few reviews on 6moons, most recent one here: http://www.6moons.com/audioreviews/a…e5/system.html
The USB DAC was introduced a bit later and it’s basically same circuit with exception of input interface which converts USB signal directly to I2S.
The DAC circuit board fits inside 2 x 3 inch aluminum tubing.
I liked the DAC in the open though and over the years few different versions of such assembly circulated.