My guess is that they are pre mix stems. Music L & R, FX L & R mono dialog and narration. That is a standard pre mix stem config. They might be Pro Logic surround compatible.
If you just put them together, they will probably not make the mix as they should be undipped for re versioning. If you assign them to tracks in a 5.1 AC3 they will be a total dogs breakfast.
If they were a 5.1 mix they wouldn’t be labeled music, FX etc .