JCV files are release version dependant: NT JCV doesn't work along NTU map and viceversa.
1xxx series receive NT version from Express or Mapupdater so will use NT JCV.
4x/5x series receive NTU so will use NTU JCV, 40/50 use their own NTU 40/50 JCV file, they wont work with other nuvis JCV and wont work with NT JCV.
New Nuvi series since 2012 use CNE NTU maps, older models CNE NT, along their version release addition files.
You can check in your nuvi for files names and, to find the right JCV file name, increase of one set of ten in the last digits per version.
Example: D2504140A.JCV -->D2504150A.JCV