I do not actually see one in your pictures but a quick release for the seatpost would work in the same manner as the ones on your wheels. now I agree wanderer that the BOLT on your seatpost clamp should go where the slot in your seat tube is.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk