Hi Gang,
I received the battery I ordered from DigiKey (BR-2/3AE5SPN) as suggested in this thread as a nice drop in replacement. I installed it today and can say it worked great. The battery is meant to sit on a PC Board, so the leads are just long enough to go through the board and be bent back somewhat to remain on the board for soldering.
I had the 3478A plugged in and powered on during the replacement connected to a UPS. My soldering station is not connected to earth mains, so no worries about shorting out the battery during replacement, As a sanity check, I checked the voltage on pin 22 of the ram chip (U512) prior to battery removal and verified it was receiving power from the 5 volt rail and NOT the battery. I did not bother with soldering on a backup battery since I was using the UPS to power the meter. I was VERY careful. After replacement, I powered off and back on, all is good!
My meter was made late 85 and probably left the factory first quarter 86. I removed the original battery (Eternacell), so it was in the meter for 34 years!! It still measures 3.02 volts out of circuit. It certainly was time to change it out!!
My next step is to replace the 4 Rifa caps on the AC input. Those are ordered and on the way.
Mike