Following report of communist, letters must show these (9)

I believe the answer is:

postmarks

'letters must show these' is the definition.

'following report of communist' is the wordplay.
'following' becomes 'post' (eg post-war, post-industrial etc).
'report of' shows a homophone (sound like).
'communist' becomes 'marx' (I've seen this before).
'marx' is a homophone of 'marks'.
'post'+'marks'='POSTMARKS'

I've seen this clue in The Guardian.

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