Based on this product's specifications, here are some alternatives and similar models
Nut cap (matte gold) for Robert Welch pepper/salt mills, size S
Summary description:
Replacement matte gold nut cap designed for Robert Welch pepper and salt mills, model fit labeled S. Serves as the fastening and finishing element for the mill top, compatible with S-size grinders and intended for use with salt mills. Provides a decorative and functional cover for the adjustment/fixing mechanism.