The Tektronix 017-053 is a 50 Ω to 125 Ω, 1 W resistive impedance matching adapter with GR-874 connectors.
The 017-052 is the same with a 0.5 W rating, as is the 017-055 with 2 W.
It was shipped as a standard accessory with the 519 gigahertz scope that had 125 Ω
50 Ω GR-874 connector
125 Ω GR-874 connector