These are UK manufactured automatic retracting (INERTIA) type seat belt. By being made in the UK they are very similar to the original style that was used on.
Inertia Reel, 3 Point Seatbelt| We have finally found a source of quality American made inertia reel seatbelts that are , MGB, Midget.
Modern 3-point Retractable Seatbelt| Black 3-point seatbelt Strapping on a seatbelt Minilite Style Bolt-On Wheels, Anthracite, 13"x5", Sprite, Midget.
For sale is a complete and working set of original Kangol seat belts for MGB. MGB GT and MG Midget.
Not sure if they fit all MGBs, the mounting bracket is a little different, but the receivers are very similar to MGB's. The seat belts are in working condition.