For supporting of formwork in constructing buildings, use is made of steel post shores (props) having a pin 15 with recessed areas thereon. When the prop has to be brought from the loaded to the unloaded position, such pin is changed of position in the prop so as to allow the inner tube to be lowered within the outer tube of the prop. The invention provides a new pin-design such that after assembling the prop it can be clearly seen wether the pin finds itself in the correct position or not. For that purpose, use is made of a pin (15) having a recess (27) and a flattened portion (29); by axial displacement of the pin this portion rests (in fig.2,10) on ring (12) or on inner tube (3) under changing its angular position, which is well visible for the user. Alternativily the cylindrical pin (35) of fig. 4 has two cams (39, 41) resting on ring 12; by axially displacing this pin, it will rotate over 90 DEG and the inner tube (3) will be lowered in the outer tube (1), which position is clearly visible for the user.