If this is very rare, is it potentially worth anything?
Thanks for your approval.
Rare: In the sense it is not listed in the Internet's largest British postmark database.
Worth: Value is nebulous, its worth to a dedicated postmark collector would be varied, in an Australian context rare pmks can fetch $5 - say $200
Worth to dedicated students of Pmks and their history is enormous, but it doesn't then necessarily translate to $ terms.
My route would be to keep it with written details alongside.