Does the plus models Nsync differ from csync in a way that I should care about when building an RGBS cable to plug into my upscaler?
If it is different: what is it, and why is it?... It's a very hard term to web-search, thanks to late 90s pop :(
No difference. It's usually just called "sync": https://www.cpcwiki.eu/index.php/Connector:Monitor
If you got it from the schematics, then it's called nSYNC on the Plus and
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SYNC on the CPC. The bar and the n probably indicated that's an active low signal.