Assuming this is part of the wall form (banche), then it is likely to be an end stop:
end stop (1) Concrete formwork at the end of a wall or at the edge of a slab. If it forms a construction joint, it can consist of drilled, or split and notched, timbers that allow reinforcement to pass while enabling easy stripping, fine steel mesh which is left in place, etc.
[Scott/Penguin Dict of Bldg]

Of course steel wall forms are rather more sophisticated than implied in the above definition. I assume the "portillon" and "barrette" are two different means of securing the stop end; if your wall form is 4 metres long and you only need a 3-m-long wall, the means of securing the stop end in the central section of the form may well be different to those used for securing it at the end.

However, "about" might also be the end of the piece cast in the form:

about - Extrémité de toute pièce destinée à être jointe à une autre [see comment about construction joints above] ...

abouts de mur, de cloison, de plancher : lignes de jonction de ces ouvrages avec les façades et les pignons.

In prestressed concrete beams, the "about" is the "end block", that part of the component in which the stress pattern is complex and severe because of the anchorages.