Mapupdater have a three variant at least (after dash record in GarminDevice.xml):

1) D3305060A.JCV - ...<DataType><Name>PreProgrammedLaneAssistFull</Name><File><Specification><Identifier>JCV</Identifier></Specification>... or ...<DataType><Name>PreProgrammedLaneAssistFull</Name><File><Specification><Identifier>JCV</Identifier></Specification>... (usually for old Nuvi and earlier version of SW in 37xx without support Unicode Map)
2) D3527040A.JCV - ...<DataType><Name>PreProgrammedLaneAssistPortraitFull</Name><File><Specification><Identifier>JCV</Identifier></Specification>... (usually for Nuvi 2xxx-3xxx especially with dual-orientation screen)
3) D3528040A.JCV - ...<DataType><Name>PreProgrammedLaneAssistTemplatePortrait</Name><File><Specification><Identifier>JCV</Identifier></Specification>... (usually for Nuvi 30,40,50)

So file in your device will updated to according with record in GarminDevice.xml what is corresponding to ...<Identifier>JCV</Identifier>...

Anybody can check for <Identifier>JCV</Identifier> in GarminDevice.xml or try to use any type of file...

P.S. But Navicom often mixed various JCV format (for dual-orientation and usual) in one file and the improper part of it look curiously on some device. It is normally for it!