OK, for some reason this came into my mind. ISTR seeing in ads or catalogs a large Neptune, around 1/32 from balsa & tissue back in the '70s. Early - mid '70s. I did a Google & didn't find any info, so, do any of y'all happen to remember anything like this?
I believe I am familiar with all the old Guillows kits. My first completed stick model was a Guillows. I do not remember a Neptune. Guillows and Comet were the two major mfgs of these kits, but there were innumerable other, smaller mfgs. So it is indeed likely that there were kits, but I suspect it was from one of these smaller, maybe even local, mfgs. There IS a neat web site for old balsa models, but I cannot seem to find it at the moment.
There is also a Yahoo group for Guillows models. I would join it temporarily and ask about the kit. Even if it was not a Guillows, these folks are stick model afficienados, and may remember it.