The used 24 x 36 Black Flag from Matthews has a rectangular frame with a 3/8 top pin for mounting on a grip head.
Matthews 24 x 36 Flag
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Matthews Photography Light Control Flags & Cutters - Buy at Adorama
Matthews 169062 Overview - 24 x 36 Black Flag for Light Control
Matthews - VISUALS e-shop
Matthews 24x36 Solid Black Flag #169062
Used to protect the camera lens from flares - Control spill light from other sources
Matthews 24X36 Solid Black Flag
Lighting Flags & Scrims
Light subtraaction and black flag/ panel recommendation: Studio and Lighting Technique Forum: Digital Photography Review
Matthews Photography Light Control Flags & Cutters - Buy at Adorama
Matthews 169062 Overview - 24 x 36 Black Flag for Light Control