I took some measurements of the broken holder using calipers and sketching the result on paper. To produce the data for 3D printing I then made a model using Autodesk Fusion. You can obtain a free version for non-commercial use. I produced a STL file from this model.
I have an old English car, nearly 40 for years old as it happens. While many parts are available new from specialist suppliers, some things are impossible to get hold of. The costs of remaking the tooling is just prohibitive for the small quantities likely to sell.
There is a nice system to fix the seatbelts in the car when they are not being used. I think this makes the car look neater when its parked and also is practical. The holder is part number ZKC1550 according to the parts catalogue. The holder is made up of a spring, a backplate and a plastic molding.