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 more than half of the new mainframes' speed; the Type A, Type B, and Type C upgrades 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, with Type L replacing it.
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. (Exception: the 7A16P, which has two input connectors and provides the ability to select which one is fed to the amplifier.)
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.