I opened up my AT&T PC6300 Plus because the CMOS wasn't holding the settings and discovered the barrel battery was leaking. I took it out cleaned up the area where it was and thought maybe if I couldn't find a replacement, maybe I could build one. I took off the wrapper and there were three button cell batteries held together with some kind of conductive paste that had crystallized over time. There is number that appears to be 638 on the bottom and the cells are made in Germany (so possibly there is a European numbering scheme that has since been replaced that I am not able to find in searches). On the wrapper it says Superpila NCB6ZA/3 +12/84+.

Does anyone know if there is either a replacement barrel battery that will sub for this or cells that I can use to replace the cells inside it? There is no place on the motherboard that was set aside to mount a coin cell holder and wiring a AA holder will be awkward because of how the case is designed. Has anyone replaced a battery in these before?