Hi Stig,
The Nieuport serials certainly are a headache.I wonder if the issues with the Nieuport 11 and Nieuport 16 might have been related to engine availability which might account for continued Nieuport 11 production even after the introduction of the Nieuport 16. Pure speculation of course. Also given the state of the records unless once has a good view of the serial number of an aircraft identity mistakes may as you indicate be a factor. This doesn't really help with the Nieuport 17 in the 13xx serial run though. I have N.1364 and possibly N.1372 as Nieuport 17s.Presumably the early Nieuport 17s came off a seperate assembly lines and numbers were assigned as machines were completed. Seems bonkers though.
This problem doesn't seems as marked with other french manufacturers in my opinion. Thus while it appears that small numbers of Spad S.XII for example might be preceded and succeeded by Spad S.VII there is not the same scattering of indivudal aircraft types as seems typical for Nieuport.
Meanwhile here are some sadly somewhat disfigured images of Nieuport 12 9238 of the RNAS:
https://www.ebay.co.uk/itm/WW1-RNAS-...EAAOSwTupf29pr
https://www.ebay.co.uk/itm/WW1-RNAS-...UAAOSw~mxf29Y4
https://www.ebay.co.uk/itm/WW1-an-RN...IAAOSwDchf29kF
Regards,
Clint