Unusual Vintage 1970s newest Sterling Silver 925 Solid 'Extender' Textured Bangle

This is a beautiful item, a vintage silver bangle with newest an unusual extender mechanism. It measures approximately 10 mm thick, and has an inner diameter of 59 mm. It weighs 16g. It has a full English hallmark, dating it to Birmingham 1973, and a makers mark R&W for Rigby & Wilson. It has been acid tested to ensure purity, and is in excellent condition, with no scratches or breaks and a fully working clasp.