Any hitchikers out there?

As I left the 'ton last night the bar maid who was putting the chairs on the tables handed me a brown A4 envelope. Regrettably it wasn't a bung for me to throw my next squash match, but was addressed to John P of Welling, with a ZZ9 Plural Z Alpha stamp on the back.

Should I take this to the BSFA AGM tomorrow or just post it at lunch time?

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Anyone know anything about "{C5}"?