On older Glastron's and Glastron Carlson's the seat base's were attached to floor by angled aluminum plates.
The forward seat pad was attached to seat base and the other three were on hinges so they could fold down to bench, with aluminum legs to support rear facing seat pad.
Yours looks to have aluminum channels for seat pads and backs.
Can you have someone hold seat backs and rear pad up so you can disconnect forward seat pad ?