IIRC the only non reflective plates can only go on cars registered prior to 1st Jan 73.
An update on this, and interesting it is too. I'm buying a Velosolex from a chap I know, and I'm looking at registering it. The MOT exemption was originally for pre 1960 vehicles, but in 2015 became a rolling 40 year exemption to bring it into line with the free car tax exemption, provided the vehicle is registered as a "historic vehicle". The Velosolex is a 1968 machine, so all is cool there.
However, the really interesting bit is that from May 2015 the number plate regs were harmonised with the 40 year rolling exemption, so as along as the vehicle is registered as a "historic vehicle" it can wear the black and silver plates. The exemption ticks round each year on 1st April, so currently applies to vehicles first registered prior to 01/04/1978 - vehicles from before that date can now wear the black and silver plates.
Interesting, eh?
As the Velo has never been registered I have to faff about with Nova etc, but at least I don't need to fanny about with an MOT, although I will ensure it is 100% safe before unleashing it's 0.95 horsepower.