Tax stamps: The letters KT which are often found on early pressings of British records is a tax stamp. This tax was a purchase tax that was imposed by the British government on the manufacturers of records, and was based on wholesale prices at a rate of 27 1/2% in 1967 (it was increased to 50% on March 20, 1968). The purchase tax was replaced by a value added tax (VAT) on March 31, 1973. The KT tax stamp can be found in the runoff grooves of singles from 1967 and 1968. A record with a KT stamp indicates that the tax was paid. Beginning in 1969, tax stamps were no longer put on records although the tax was still imposed.
from site:
https://pinkfloydarchives.com/DUK45NM.htm