The Tektronix Type D is a differential amplifier plug-in for 500-series scopes.
It has high gain, and the bandwidth depends on the gain, ranging from 2 MHz at 50 mV/div and up, to 300 kHz at 1 mV/div.
It displays the difference between two input signals, with a common-mode rejection ratio of 10,000. It has six tubes, on a shock-mounted subchassis to avoid microphonics.
Type D was introduced with the 531 and 535 in 1954 as Type 53D, was briefly renamed Type 53D/54D in 1955 with the introduction of the 541 and 545, then quickly renamed Type 53/54D, then shortened to Type D in 1959 with the introduction of the 5xxA scopes. Its last year was 1968, having been superseded by the constant-bandwidth model 1A6.
Some production runs used a pair of 5814 tubes at the input while others used 12AU7's. These two tube types have different heater currents, and because the heaters are series-wired, you cannot switch from one type to the other without circuit modification.