K
The Tektronix Type K is a plug-in for 500-series scopes.
Max sensitivity is 50mV/div, and bandwidth in a 545 is 30MHz.
Type K was the fastest of the single-channel plugins. Tek did not equal it until Type L in 1957, and did not exceed it until Type 1A1 in 1964. Earlier single-channel plugins 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 plugins would use A-J and fast ones L-Z. This idea never went anywhere.
Type 53K/54K was introduced in March 1955 along with the 541/545. At that time, it was the only plugin 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 5xxA mainframes came out, it got its final name, Type K. It was dropped after 1970.
Late versions had a BNC connector instead of a UHF connector.
Types K and G are the only letter-series plugins where the Gain vernier is not concentric with the step attenuator.