a fitting 28 possessing a slot for passage of the shoulder strap of seat belt 7 fitting 28 is mounted for rotation by means of two part blocks 29 29 39 on an axis 30 screwed through space washers into nut 31 soldered onto a return plate 35 rigidly connected to flexible rack 25 so as to immobilize anchorage 6 in position particularly following a traction force applied to the belt the latter bears a clamp brace 32 engageable between the teeth 34 of a terminal sheet metal part 33 supported by the roof post the terminal sheet metal part 33 extends along an adjustment band for the movable anchorage the apparatus may be equipped with a system for adjustment of the stop position of movable anchorage 6 when the door is closed and motor 26 stopped the system shown in fig4 is composed of terminal part 33 and a guide protrusion 40 in which moves a catch 41 carrying end of track contact 27 and a threaded shaft 42 threaded shaft 42 traverses a guide aperture 43 supported by the roof post a tightening nut 44 assures the locking of catch 41 in the position desired by the user this arrangement thus makes the immobilization position of the movable anchorage dependent on the position of an end of track contact and enables the adaptation of the use position of the anchorages to the body characteristics of the user of the belt fig8 shows a power circuit for direct current motor 26 with permanent magnet and equipped with a reducer the latter possesses a recocking thermic circuit breaker 210 the plus and minus poles of the motor are connected respectively to the terminals of the contacts of part c of the door the opening and closing of.