Plastic ceiling rose made by Volex. Probably dates from around 1960.
This example is rather dirty and yellowed. Originally it was white.
The main part of the rose has two fixing holes 2 inches apart. This would have fixed directly onto a round conduit box, with any additional connections being in the conduit box, rather than on the rose itself.
The round ring fits between the rose and the conduit to cover the edge of the metal conduit box and give a neat appearance when fitted to the ceiling.