Well, about the "Nieuport 60" I've found that page
http://aviadrix.blogspot.de/2012/02/les-series-experimentales-de-chasseurs.html,
more interesting, than just the mentioning of this type, may be what is said about the designation system .... :
You're right, that "NiD 60" is used twice indeed, but it's still the same source, the only source, so maybe based on the
same error ? Don't get me wrong, I'm absolutely no specialist for Nieuport aircraft and from all we know until now, it
could well be a double use. There's the logic in the "updating" of the number (50 for the twin-, 60 for the four engined type),
cutting off the last digit of a type number, or better expanding the designation by an additional digit can be found with other
types, too and as the "NiD 60" actually wasn't used (this flying boat remained a paper tiger), why not use it again, for an
aircraft, that actually took to the air ?
Nevertheless, we cannot take it for certain and I wouldn't recommend taking a single source as evidence. I really like Gérard
Hartmanns and I'm still hoping for more, but even his work isn't completely free of errors.