I don't remember any mention of Cambodia in the MSP before the NYT publication of the "Pentagon Papers"
re: Pentagon Papers--
NYT published on June 13. 1971, according to this link:
http://www.jfklancer.com/pentagonpapers/
The link also notes that the papers were provided to the Senate Foreign Relations Committe in 1969 (no date specified).
My recall, admittedly shaky, is that the PPs didn't become news until the NYT published them.