Silver metal rectangular ring 38mm.
The ring can be used with the adjuster to make a cross body bag strap adjustable! - https://www.owlandsewingcat.com/collections/haberdashery/products/metal-rectangle-adjuster-used-for-the-lucielle-bag
Lucielle bag sewing pattern can be found here - https://www.owlandsewingcat.com/products/lucielle-bag-pattern