IBM ThinkPad R31

The IBM ThinkPad R31 supports a maximum of 1GB of RAM, spread across two memory slots. It uses PC133 SDRAM SO-DIMM modules. You can find compatible modules from various vendors, often in 128MB, 256MB, or 512MB sizes.
Details:

Maximum RAM: 1GB
Memory Type: PC133 SDRAM SO-DIMM
Slots: 2
Upgradable: Yes, up to the maximum capacity of 1GB
Common Module Sizes: 128MB, 256MB, 512MB