My first DAC was the Black Box 3 (it had one more button than yours) which I bought in 1991, together with a Luxman drive which I got for half price because nobody wanted it. It came with a Luxman preamp with DAC included (probably one of the very first) which was already sold.I can't remember the exact model (maybe the 5?) but my first DAC was an Arcam Delta Black Box looking something like this:
I bought it second hand in around 2002, with a Delta 250 transport.
I still have it and did a few measurements. The BB3 has one of the first delta sigma converters if I remember correctly (have to look into the schematics). Anyone interested in measurements of vintage stuff like this?