IBM 5155 Manual To Operations page 330

Specific Instructions
The system board of the IBM Portable Personal
Computer comes standard with 256Kof total memory.
You can only install the memory modules on a
64/256KB Memory Expansion Option.
To install the modules on the 64/256KB Memory
Expansion Option, go to the Option Instructions that
came with that option.
2 64KB Memory Module Kit
January
1984