Also see PRS, Oslo
This was just another name referring to the same pressing plant (Musikkindustri A/S).
The name Polygram Record Service, with or without Oslo at the end, appear on releases for manufacturing or pressing.
If Polygram Record Service is credited on releases with "Pressed By", credit both Musikkindustri A/S.
Use "Manufactured By" or "Made By", if credit is a more generic one, like "Fremstilt" or other similar Norwegian words meaning company was involved in making the release.
If just the name appear without any role specified, use the catch all "Record Company" as the credit.