Do you remember a TV programme which brought puzzling things in for people to solve? I think Frank Muir may have been on it.
I am fairly sure this (original picture) featured in one - or at least it looks like it.
When you are repairing chair seats with webbing, as they used to in the old days, you pull the webbing through this object, then brace it agaisnt the side of the seat, pulling to make the webbing taut, then hammering in the tacks before letting go and trimming the webbing.
Does that make sense?