It was a close up device : a screw mount additional lens for Elmars 50mm "new design" (3,5 and 2,8 with E39 filter thread) which permitted focusing from 20 to 34 inches, and a rectangular lens, fitted by an arm to the shoe which adjusted the VF/RF image. So, better than the old simple ELPRO/ELPIK/ELPET etc lenses for the old Elmars, which lack anything for VF/RF "coupling", but no better than the NOOKY etc, which used the tech of extension ring to close focus, that gives, in general, better image quality vs, additional lenses (nad were anyway VF/RF "coupled"). This is the reason for, I think, ADVOO didn't have great success... and the Leica SM time was at its end (M3 was already on the market for 4 years when ADVOO was announced), so it's surely rare: an italian shop (newoldcamera) has one for sale at 250 Euros, Tamarkin one at 275 US$... maybe others can be found, but surely it isn't common. Of course, it's strictly for IIIG, given that the VF lens has to fit the shoe and stays on the lens (it is gently curved to couple the Elmar fron ring): everything is dimensioned to IIIG body.
For the record, I seem to remember that a similar device was made by Voigtlander for the Prominent, and maybe also by Zeiss for some compact 35mm camera... don't remember if for the Contax too.
You can find pics of the ADVOO in the usual books (Lager, Laney...)
<added after posting> ... or , thanks to friend JC
... here above...