Update: I was able to source the item.
Looking for 33120-80001 Opt. 001 Phase-Lock Assembly for an Agilent 33120A Function/Arbitrary Waveform Generator. These seem scarce. Nada on eBay or anywhere else I could find.
If you have a dead 33120A that you could pull Opt. 001 out of or one lying around otherwise, I'm willing to buy.
— Jeremy