Nice find Jamie, haven't seen this add either, EffCee may be on the right track, there were a few different makers of these adapter blocks
I was under the impression that they were from the late 60's early 70's but the add proves otherwise
I know my Datsun 1600 had a dual throat carby & it performed just like the add says only mine was in dual throat mode most of the time
Here are is a pic of a unit I have, it has Nikki on the side which I think is a brand of carby
Does anyone know by the shape what carby would fit this adapter
and another angle