The 5161 Extender card will work in the 5150, 5155, and the 5160 64-256k board but not the 256-640k board. (if your 5155 was upgraded to the 256-640k then = SOL)
Are you sure about the 5155? As far as I know the 640KB upgrade is an unofficial mod, there is no official 640KB IBM 5155 MB. So why would IBM list it as incompatible w/ the 5161?