When I was reading this thread, I found myself agreeing with brianj42, go into filling station, hand over 67 leva and get vignette because that is what happened to us (not asked for registration number), though I do know that Picadilly have always taken the registration number for their own records.
I also thought haddockman was totally wrong, for which I now apologise.
What I did notice when I bought it was that it is larger than previous years.
I have just looked at ours and at the top is a space and underneath that space it says 'Area for registration number'.
Too late for us as it is already stuck on the windscreen.
Wonder how many other people have done this without looking as this is the first year it has been like this?