I realise cars issued with plates prior to 1988 were issued with these as standard.
The reason I ask is I saw a '88 Toyota Corolla stationwagon today that had a recent number plate (FLN7XX - i have removed the last two digits just in case) that was exactly as above, sliver letters on a matt black background.
I didn't get a good look at it (since I was driving) so couldn't tell if it was home made or not.
I have done a quick search online, and found the following forum that has links/exerts to/from the Transport (Vehicle and Driver Registration and Licensing) Act.
What are your views, looking at the email that one of the forum members was sent, it does look like either option is legal.