These are brand new countersunk thin plate style 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 to be used as a guide.
• Quality: All products are of the high quality and meet the relevant international standards.
• Stock:  We carry a large inventory of stock in Sydney Australia,  so all orders will be sent within next business day of your payment.
• Postage: Your order will be posted with Australia Post.