Looks like a captive nut rather than what I know as a rivet nut. To me a rivet nut is a nut that is expanded (Riveted) into a hole to form a captive nut. This one appears to have 2 rivets to anchor the nut to the structure, quite a common type of fastener for those applications that you can't get a spanner to the back of.
Thanks for your help.
It is indeed an anchor nut, still commonly used throughout the aviation industry . the nut you have has is whats called a capped nut ,whish means the screw/bolt does not go out the other side to prevent leaking . It also has steel rivets which means it would have been used in a hot area ie an oil pump etc .
thanks for your help.