From Colin McDonald, "The osgb file reader uses seek to position within the file.
But the nested stream implementation for osga archive files doesn't
support seeking.  So osgb files can't currently be used in an osga
archive e.g. if osgdem is used to output a osgb format database it
can't be packaged in an archive file, in the same manner that ive
files could.

I've added seek support to the osga nested stream implementation."