In the post-war period all luxury goods, including records, were subject to 'purchase tax'. Items produced before and during the 1960s were subjected to different tax levels that varied from year to year. Records have a tax code pressed into the vinyl either in the deadwax or embossed into the centre section of the record label. There are some crossover tax codes on labels such as 'MKT'. The reason for this is that during transitional periods (for example July 1963) EMI would have made up a number of pressing plates which would have had to be scrapped as the wrong tax code would have been on it. As the last letter of the tax code was always a 'T', the correct first character was added to the stamper and used until that stamper was discarded. Hence why 'MKT' is a possible tax code - 'MT' was the older one with a 'K' being added in July 1963 indicating it to actually be 'KT'.
The following chart gives the dates of introduction.
Date of First Use
10th April, 1962
26th November 1962
1st January 1963
1st July 1963
1st January 1969
Its use was removed in late 1969.
Stolen from here: The Beatles Collecting Guide