The Tektronix Type K is a plug-in for 500-series scopes.
Max sensitivity is 50 mV/div, and bandwidth in a 545 is 30 MHz.
Type 53K/54K was introduced in March 1955 along with the 541/545. At that time, it was the only plug-in that could utilize the new mainframes' speed; the redesigned Types A, B, and C weren't ready until August 1956. In August 1955, just five months after introduction, it was renamed Type 53/54K, and, in 1959, when the 541A/545A mainframes came out, it got its final name, Type K. It was dropped after 1970.
Earlier single-channel plug-ins had two inputs and a switch to select between them, making it convenient to probe two test points at once. The Type K design could not tolerate the extra stray capacitance, and the feature was dropped, never to return.
The name was deliberately chosen to be halfway into the alphabet. Slow plug-ins would use A-J and fast ones L-Z. This idea never went anywhere.