How was this bench upholstered?

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I picked up this bench for $20 at Salvation Army. It's nothing special, seems to have been built relatively recently but it was in decent shape and I thought I could salvage it. The thing is I can't figure out how the seats were attached to the frame.

When I bought it there was a border of old material that was stapled and glued to the top of the frame. I pulled out the staples and removed most of the material but you can see the stray threads where there's still some glue. There was also a plywood square in each recess in the frame but it seems a bit flimsy to support one's weight. There doesn't appear to be any diagonal supports for a seat to screw into like I've seen in most chairs. Any idea how this was originally constructed?? Thanks for your help!

-Carl