First the first three years of Discworld stamps there had been a Cabbage Fields 50p. These were not considered part of the AMPO definitives, because they were not issued with those, and the first one was only available to collectors on the Waterstones cover and as a free gift with issue 1 of the Stamp Journal. For the Year of the Three Roses however, the 50p value was a green Tower of Art stamp slightly reduced in size compared to the $1 value. This was because in Making Money Terry Pratchett wrote ‘Mr Spools unlocked of of the lower drawers of his desk and tossed a sheet of 50p green ‘Tower of Art’ stamps onto the blotter”. The stamps in question were Owlswick Jenkins forgeries, but there had to be such a stamp for him to forge.
Thus they were released in sheets of 28, without any sports. They differed from the Year of the Prawn $1 stamps only in terms of the value being replaced and a resizing.