The Yaesu YC-601B is an external digital frequency display for the FT-101/277 and FT-401/501 series.
It shows MHz, kHz and 100 Hz via six red digital display tubes.
Unlike the earlier YC-601, the YC-601B also features a built-in frequency counter covering 100 Hz to 35 MHz.
A BNC input jack is on the front panel for the counter.
The YC-601B requires 100/110/117/200/234 VAC 50/60 Hz.