Complex sentence: For philatelic purposes such states were referred to as "convention states" as opposed to "fuedatory states" which had no postal agreement with British India, which issued their own stamps, the postage validity of which was within the borders of the state itself.
Split into simpler sentences: For philatelic purposes such states were referred to as "convention states" as distinct from the "feudatory states" which had no postal agreement with British India. The feudatory states issued their own stamps, the postage validity of which was within the borders of the state itself.