The DS1287 is 24pins. It appears that the DS1260 is not a RTC/CMOS it is just a battery and a controller to connect/disconnect the battery from the circuit.

Datasheet:http://pdf1.alldatasheet.com/datasheet-pdf/vi … LAS/DS1260.html

I don't know what revision you have but is the external battery header at j6 populated, based on the revision that I have seen, http://www.amoretro.de/wp-content/uploads/opt … 4167_weitek.jpg , J6 is located right below the second screw hole down on the left side(assuming the top left corner is where the AT keyboard connector is).

To be honest I'm kinda surprised that the battery is dead since the DS1260-100 has a 1000mah battery in it , and the cr1220 batteries in in DS1287s are only about 50mah and they tend to last 15-20+ years, CR2032 batteries are around ~200-250mah and in most modern computers I've seen them last a good 15 years as a CMOS battery and I've heard of them still being good as SRAM batteries in game cartridges after ~25ish years so you'd think that a 1000mah battery would still have a good amount of life left in it as a 486era CMOS battery but I guess it may depend on how the board was stored as elevated temperature can have effects on self discharge rates.