Ah, OK - it wasn't clear how that was supposed to be accessed; so if that red piece with the slot is visible, then it's likely the part in question. I approve of the fix, actually - anything which can render the equipment non-functional by accidental loss or breakage should not be a unique part. I got a piece of old HP test gear last week with a broken fuse holder and it was literally 5 minutes to restore it to as-new functionality with no hacks because it was a standard panel mount component.
I'm hoping to find a used 3468A or 3478A which is in repairable condition and this will help!