I emailed Rina at ORRA, asking her about that, and this was her reply:
Has anybody done this?ORRA don’t issue the letter stickers mentioned. On the licence received you will find your allocated call sign (X88). This call sign ,must be displayed on your vehicle. The recommended size is +- 75mm black on yellow sticker available from your local hardware store.
What kind of stickers are they? (pictures please)
Where can I get that blue ORRA sticker? I thought that was the one I had to display....