The latter one is a real first day cancel and an official one.
Amateurs may think that the first one is just a private FDC, specialists know it is bogus. The clue is there: the French "CAMBODGE" type postmark did not exist in 1980s, it is impossible for it to be on a 1983 cover.
Below is the postmark type used by the Phnom Penh Central Post Office throughout 1980s, the country name was "KAMPUCHEA" back then:
Private FDCs of 1980s would only bear such cancellation type or the pre-1975 "Phnom Penh RP" with no country name type.
Cambodian postal workers have been known for their bad practices and one of them is postmark back-dating which actually prohibited by UPU. Back in the 1960s and 1970s, just a little bribe could let you have postmarks of any date you wanted. This "tradition" lingers till today, consequently it weakens the credibility of certain postal and philatelic materials. The other side of the coin is, it keeps amateurs entertained and specialists busy, you complain and yet you enjoy it.