While it may be possible that the plates were postdated, there is no evidence of such fact at hand.
It might have been that the article was based on speculation, and it might have been the newspaper was postdated too.
also post-date, 1620s, from post- + date (v.1) "to assign a date to, to mark a date on." Related: Postdated; postdating. Intransitive meaning "be of an earlier date" is from 1909.