keeper hook

Also known as a Batten Hook (by Mutual Hardware at least), an S-shaped piece of strap iron used to temporarily attach a stiffener to a Boardway flat's toggles, in order to straighten a long run of flats.
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http://www.mutualhardware.com/Theatrical-Set-Hardware/Stage-Hardware/Batten-Hook

See figure 88:
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Another variety provides for a 1x3 or 1x4 stiffener to be used on edge, making for a much stiffer wall. Loose-pin hinges, alternating top and bottom, are often used instead, but take more time to set-up and strike. This type of hardware is often shop-made rather than purchased, so many variations and names exist.