A folding pillar (10) is provided with a rotating locking assembly (34) which includes an eyelet (28) for the passage of a sealing cable (26), as well as a locking member (42) fast for rotation with the eyelet (28) and adapted to interfere with a lateral abutment (32) of a bolt (30) incorporated in a drop board to prevent the bolt (30) from leaving its position of engagement with a bush of the board when the locking assembly (34) is in an angular locking position which corresponds to the passage of the cable (26) through the eyelet (28), whilst the locking member (42) does not interfere with the path of the abutment (32) when the locking assembly (34) is in a different angular position from the previous one. The locking assembly (34) is rotatable about an axis which is normal to outer and inner walls of the pillar (10) and includes, within the pillar (10), as a locking member, a substantially circular-sector-shaped plate (42) with a circumferential edge (52) which is adapted to interfere with the lateral abutment (32) of the bolt (30) when the locking assembly (34) is in the locking position.