I've recently bought a Nettar 516/2 as well, on an internet auction, from a woman who got it from her granddad.
At first I thought it was a whole other type of zeiss ikon, since I couldn't find anything on my actual camera. The only thing on the internet I found was that it's supposed to be from 1938, which I guess is just a mix up with the 515. I was quite extatic finding this topic, because it's the first actual correct information, I guess.
I was wondering if you could send me the Historica pages as well, if you still have them, because I'm curious to find out more about it.