Here's the 'missing' and very much unheralded 1st Class amethyst from Counter Sheets.
The background text features the word (REI)GN at the top of the security slit on the right. That is hardly a 'code' because it is how we usually spell the word 'reign' but it is the simplest way to identify where an individual stamp originated as the others will have the different codes or spelling shown in a recent article.
I think this particular stamp has to be one of the least publicised 'normal' issues of all time. I can vaguely appreciate why Royal Mail took the decision not to issue details in advance generally to the public but I am still surprised that this sheet stamp was excluded from all the official lists that I have encountered, Nor did it get included in the Philatelic Bureau's new issue or even their 'definitive changes' distributions.
So I have a suspicion that quite a few collectors may miss this if they simply rely on Philatelic Bureau or Royal Mail publications.At the moment, they'll either have to visit their local Post Office or buy one from a dealer. I can't imagine that situation will remain the case and eventually the sheet stamp will appear in Royal Mail stock listings but by then some will just think it is something they already have.
Visiting a local Post Office is not a bad idea anyway. One reason is that you just pay the face value of the stamp, of course, but I tend to buy one of each of whatever definitives they have in stock from time to time and then compare them to previous purchases. More often than not there's nothing different but just occasionally I will find an interesting shade or variation that makes it worthwhile. I can usually use the duplicates as I post a lot of parcels but you may prefer to keep to the lower denominations. Because they get reprinted more frequently they are more likely to have something different anyway.