IBM 5161  -  5160 models "not supported"

The 'IBM PC Family Service Information' document includes the following in the 5161 section:

"Note: This unit is not supported on 5160 Models 088, 089, 267, 268, 277, and 278."

Those were the later 5160 models, the ones released in 1986.

There is no detail on IBM's meaning of "not supported".

Initially, I suspected that this was BIOS related, because in the move from the 1982 dated 5160 motherboard BIOS to the 1986 dated ones, some (some, not all) of the 5161 support code was substituted by code that addresses a dynamic RAM requirement.  This is shown at here.

The substituted code is code that enables the extender card during the power-on self test (POST).  However, research revealed that the substituted code is not required; the circuit diagram of the extender card shows that the card is automatically enabled at application of power.