Option #1 High frequency stability reference.

Frequency range: Sine 21 MHz / Square 11 MHz / Triangle, Ramp 11 kHz / 60 MHz Sync

Other details see specification in the service manual www.Keysight.com/us/en/assets/9018-05130/service-manuals/9018-05130.pdf

Due to actual instability, the shipping costs my oscilate more than usual, so best to be revised before concluding the buy.