This is a combination of "poster" or "pin up" and "square."
Use your thumbs to press the top corners of an imaginary poster to the bulletin board, then press the bottom corners. Then sign "square."
bulletin-board:
Sign: bulletin-board
Synonyms:
Handshape:
Right Hand:
Left Hand:
Location:
Orientation:
Movement:
Non-Manual Marker:
Interpretations:
Inflections:
