Since this is an ''export'' it probably means it was a CVS export of the main NetBSD tree.
See: http://netbsd.org/releases/formal.html
A date of 1998 / April / 07 (this is how I read the date of the export) - so that would be a CVS version between NetBSD 1.3.1 and NetBSD 1.3.2.
Make of that what you will, since I am not a NetBSD expert (but I have installed and used it a dozen times already).
FWIW, NetBSD newest version is 6.1 - yes, 1998 was 15 years ago, and I was much younger then...