Here's the DAVPEX '83 show cover.
There's a couple of interesting details about this:
1. The canceller from the post office states "Davao Philatelic Club" although the club's name was "Davao Stamp Collectors". Not long after, however, the club did change its name to Davao Philatelic Club!
2. The name of the exhibition is misspelled on the cachet: DAVAPHEX instead of DAVPEX. I believe the printed envelopes came from the Manila post office as there's a similarly-designed cachet for LASALLEPHEX held some 10 days earlier in Bacolod City.
The stamps are denominated 40-sentimo, the postal rate for domestic mail at that time.