These are yellow zinc plated steel small thin countersink head rivet nuts;  they are knurled to grip better against rotation with following dimensions shown in the photo section of this listing:

• What you get:All measurements are approximate.
• Quality: All products are of the high quality and meet the relevant international standards.
• Stock: We carry a large inventory of stock so all orders can be sent within next business day of your payment.
• Postage:  All orders will be posted with Australia Post.