Trust me the discs are Prodisc
It's exactly the same way that Ricoh discs are made by Ritek, or the Sony media coming out cheap is Daxon (Benq).
The dye has been jointly developed between the two companies (Prodisc and Fuji) and the media will be the same as branded Fuji discs, but may be B-grade. That's not a bad thing, because the Japanese companies typically have a very high standard for QC testing and the discs probably are designated B-grade due to a very small defect, which won't affect performance.
Let us know how you get on anyways
I think you're right mind when you say that the MID will probably be FUJIFILM0x (Prodisc's equivalent is PRODISCS04 so I'd reckon these will be FUJIFILM04).