Yep, youre corrrect the seat belt anchoring point is the main differance and I have read of people trying to drill the e92 seat to bolt on the belt but the seat does not have suffcient metal structure in order to create the threaded hole. So it's a no go for adaptation.
The only way around this may be creating an anchor on the B-pillar similar to that found on the e92., however I wouldn't risk it. (safety factor at play)
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