155-0059-00
The 155-0059-00 is a Tek-made HF Amplifier chip. The Tek custom chip catalog calls it a "Gain Trim Amplifier". It's a 12 lead TO-8 package. Used in the 7000 series 400 MHz and 500 MHz scopes in the vertical amplifier section. The final vertical gain adjustment is after this amplifier and it typically leads into either a 155-0064-00 or 155-0065-00 output amplifier which drives the vertical deflection plates of the CRT.
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.