tulip reupholstery issues  

I've acquired a set of tulip chairs (armless, fully upholstered inner shell) for next-to-nothing. Former owner reupholstered them in the 1980's. I've disassembled one to determine how easily I can redo the upholstery.

I'm trying to determine what the previous owner did, versus original Knoll construction: I've found that the inside & outside shell of seat was glued together, with a thick grey glue. I've successfully pried it apart, but... were these pieces originally glued together?

Any of you have experience in reupholstering these chairs? the pattern is simple to recreate, I'm concerned with reassembling them properly.