The Tektronix 155-0059-00 (M077A) is a Tek-made HF amplifier chip, designed by Thor Hallen. The Tek custom chip catalog calls it a "Gain Trim Amplifier". It comes in a 12 lead TO-8 package and is used in the vertical amplifiers of the 400 MHz and 500 MHz scopes from the 7000 series.
Note that this IC can fail by becoming non-linear. It will continue to amplify but will show non-linear gain and clipping when using a step calibrator.
According to Modification 050-0650-00, Vertical HF Amplifier Replacement, the emitter resistors need to be selected according to a code printed on the device.