This could be gland nuts on shafts and pipe fittings, or a very narrow hex on a stud or shaft designed to hold it still while another fastener is removed. Contents: 6mm x 7mm (2.1mm thick); 8mm x 9mm, 10mm x 11mm (2.5mm thick); 12mm x 13mm, 14mm x 15mm (3.5mm thick) and 16mm x 17mm (4.5mm thick), 18mm x 19mm (4.5mm thick); 20mm x 22mm, 21mm x 23mm and 22mm x 24mm (5.5mm thick).
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