1931 roadster seat cushion back mounting screws or bolts

[37 Coupe]

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This is on an original seat back cushion.The question is on the three mounting bolts or screws.I know they are 1/4" -20 threads but what head are they or how do they fasten ? Are they wood screwed on other end or somehow fastened to the seat springs on the hidden side? I have a couple snapped off at the threaded eend and trying to figure out if they can be fixed or replaced .The threaded end comes out of wood framing but something on opposite end keeps them in place.Thanks for any help.