MPO is an acronym for "Moulages Plastiques de l'Ouest" (in English: Western Plastics Castings).
MPO pressed its first commercial CD on 4th of December, 1984.
Vinyl discs pressed by MPO usually have the hand etched "MPO" inscription in the runouts.
Another identifier can be that MPO pressings have three concentric rings around the spindle hole (see this image).
MPO can be credited as M.P.O. on releases.
MPO CDs made at Averton are assigned the following SID codes:
Mastering: L031, L032, L033, L035, L036, L039, LY88 (often misread as LV88)
MPO isn't always explicitly mentioned in the CD matrix, but can sometimes be identified by a string of tiny text with the following format:
CA [label cat#] [one or more @] IFPI [mastering SID code] M/D/YYYY HH:MM:SS [AM/PM] NNNNNNNNNN
(This isn't usually allowed, but there was forum discussion for doing this for MPO in particular.) If you aren't sure, please ask in the forums.)