There's nothing about this officially (in the documentation or so), but what you can do is this:
$ strings ~/nso/4.4.2/etc/ncs/tailf-aaa.fxs|head -1
6.4_1h
$ strings ~/nso/4.5/etc/ncs/tailf-aaa.fxs|head -1
6.5_2h
You should read this as the first fxs file belonging to the NSO 4.4.x release train,
the second belonging to the NSO 4.5 release train. Remember to always subtract 2 from the major num.
