Car appreciation... the aesthetics, the engineering, etc

Are the current fixings basically plastic rivnuts?

I don't know, the plastic insert that the registration plate sits in appears to be one single piece of plastic, with the threads themselves being made from the same plastic as the rest.

Of course, being painted it is hard to say whether the screw-apertures are part of a rivnut style thing, or not. I'd have to sand off the paint to find out, which I'm not hugely keen to do.