That is correct Sir, on the passenger side there are some holes drilled vertically in the transmission tunnel but the bottom two holes have rivets in them, haven’t got underneath to see what their purpose is.
The bases came with all types of brackets that I don’t know their purpose
The two pcs of aluminum tubing with the ends squished flat are for the tunnel insulation panels.
The long silver/bare aluminum strip closes the tunnel side of the seat boxes.
The installation would have another sheetmetal flange about 4-5 inches long for the seat bucket to the rocker panel.
I also have a NOS seat base bucket, it is not pre-drilled for the pop rivets. Yours, if you have the old style A0-A1 rear seat backs/footwell covers, that can be used to properly position your bucket, using the seat back with the twist lock fasteners securing the seat back to the fender front. Best I can do is shoot a couple pics of the empty pop rivet holes in my 1995 M1025A2 GMV. Put in a similar quantity and spacing cherry rivets along each contact area, I'm not sure if AMG applied the Permatex between the seat buckets and the rockers/transmission tunnel.