This feels to me akin to Shaker sweater stretchers and the mitten stretchers I listed last week too; this one with a strong tension to the hinged metal strips held between gracefully arching and tapering pieces of pine, which would be inserted inside a sleeve and then released to pull it taut. And it's one that feels like a graceful three dimensional line drawing to me, and with a terrific warm, dry wood patina to the pine, too.
22 1/2" l x 10" w fully open, x 1/2" d. Very good condition.