I'll bet that very few side-seat owners have ever had a rider back there so while the speaker installation looks odd, it probably didn't sacrifice seating space...
Cars that came with a side-seat also came with the pieces to restore the original bench seat. I guess this car didn't have those pieces else a more standard installation could have been made...
Doug: how do you find so many strange and hidden Pagoda websites???
Richard M
'71 280 SL, Tobacco Brown, Cognac interior, Tan top, Jump Seat, 80K orig miles, NYC.