Length-convertible latch with a front box (2) inside a cylindrical receptacle (7) joined to the rear body (4) with the locking nose (6) fixed lengthwise and with a transverse cavity in the body (4) where the nose (6) can be transversally accommodated against an elastic ring (16) only in its angular position of rest and be disengaged from a first pair (9, 13) of transverse lugs of the driving arm (3) to become located in a second pair (10, 14) of transverse lugs--where arm (3) and body (4) are displaced relatively, and becoming engaged in this new position.