A Dictionary of Sheep

I was driving round the countryside at the weekend when out of the blue my visitor said, 'I wonder what the dictionary of sheep is?' pointing into a field where a flock were all sporting blue smudges. Interesting, I thought. Dictionary of Sheep. The guy's losing it. Turns out there is a dictionary of sheep. According to Adopt-a-Sheep website, every five years in the UK, "The Shepherd's Guide" is published, listing every marking from every hill farm in the UK. The markings consist of an amalgam of marks and letters. I would have thought that the markings would have been quite localised to prevent sheep grazing on common land becoming mixed-up. I didn't think they'd collate them all together for the whole UK. An example of an old guide can be found here. I can't find a guide for Ireland, but I'm sure there probably was one. It seems to be all electronic tags these days.