I was suprised at how many there are with the differing companies !!.. I know some breeds have uk on them, Also a U can mean has been microchipped... ??? Saying that i went and viewed a horse years ago for my daughter was cheap for what it was capable of !!! The owners removed the rug and it had a stamp mark of a circle with a o in the middle !! although its meant to be a L!!stamped in the middle of her side... I knew straight away what it was but played ignorant and asked them.. this was a TOP competition yard and they said they did not know !!! but because it looked like a doughnut shape thats what they had named the pony.. Got home and rang Farmkey to find its loss of use was 2 years before ... they would not tell me the reason !! but did give me the area which was local to us that loss of use had been claimed.. There are a few markings for loss of use but they dont seem to mark many anymore !!??? I know sometimes when certain breeds are imported into this country the mark them also.. Sorry if not that much help but i think its a bit of a mind field as to some freeze marks..