These are the open end large-flange style zinc plated steel rivet nuts, they are ribbed to grip better against rotation with the dimensions as 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 will be sent within the next business day of your payment.
• Postage: All orders will be posted with Australia Post.