Hy mdanh,
I lived at RAF Seletar as a kid around 1953.
As you and the other members say, the hp HP3468A is a nice instrument
Just in case you haven't got the service manual: **broken link removed**
As you say, a 3V lithium battery stores the HP3468A calibration data in SRAM, and if that calibration data is lost you have to recal, which is a pain.
But changing the 3V lithium battery is no big deal. When the mains is on the SRAM is powered by a separate mains derived supply line via a diode. When the mains is off the battery powers the SRAM via another diode.
If you are concerned about the existing battery running low, it is my understanding that all you need to do is ensure the instrument is powered by the mains and you can simply change the battery.
This is my firm understanding but, as the data in the RAM is so precious, I would double check that what is say is true.
spec