Great Question.
Its a AGDR designed O2 ODA amp.
ODA Desktop DIY AGDR Headphone Amp
Only about 5 or 6 ever built…its a powerful amp, very clean and I would stack it up against most any SS amp up to $1K or even higher…
AGDR is a wizard that unfortunately has dropped of the radar screen…this amp can be found in a long detailed thread at : Diyaudio ODA Headphone Amplifier
I never had the front cover engraved, should do that some day!!
Alex