The Tektronix Type 124 Television Adapter is a TV sync separator allowing a general-purpose oscilloscope to be triggered on a specific TV field/line.

Tektronix 124
Television Adapter
Type 124 description in 1957 catalog

Produced from (?) to (?)

Manuals
(All manuals in PDF format unless noted otherwise)

It also contains a time marker generator that can produce a marker highlight on the trace via the scope's Z input.