Silver metal rectangular adjuster.
The metal adjuster can be used with the rectangle rings to make a cross body bag strap adjustable! Rectangular rings can be found here - https://www.owlandsewingcat.com/collections/haberdashery/products/metal-rectangle-ring-used-for-the-lucielle-bag
Lucielle bag pattern can be found here - https://www.owlandsewingcat.com/products/lucielle-bag-pattern