The Tektronix SD-32 is a 50 GHz sampling head for 11800-series scopes.
From Tektronix' site:
The SD32 is a single-channel, 50 GHz bandwidth sampling head. The SD32 performance is available for any new or existing CSA803A/11801B mainframe. The SD32 has measured bandwidth of greater than 50 GHz and calculated rise time less than 7.0 ps. The front-panel connector is female 2.4 mm, and an adapter is provided (011-0157-00; 2.4 mm male to 2.92 mm female) in order to maintain compatibility with SMA connector systems.