My car (early S2) has threaded bobbins bonded into the body shell, separate from the fibreboard bulkhead. You can just see the LH one in this pic:
Based on the parts book, there would have been an eye bolt threaded into it complete with the seat belt.
Looking at the parts books for the S1 and TC, the bobbin appears to be a plain bobbin, so the bolt passes through it and threads into the tie bars.