I'm wondering what a "fake speaker" is.
The speaker in the P/N 1394540 functions as the system speaker for earlier RS/6000 computers, which makes sense if you think of having a remote console away from the system itself (and system beeps actually being useful).
As far as I was able to find out a speaker was never offered with PC products, though could well have been intended. A suggestion that it was implemented in a "MF-I" [Multi Function 1] keyboard, which I have seen repeated, was retracted by its author.
The speaker is controlled by the two extra lines that are usually redundant on a PS/2 6 pin mini-DIN, so if you were using a 1394540 with a RS6K product you would need a six conductor cable. For a PS/2 application, you won't be able to do anything with speaker, but I always wondered whether it would complain if you didn't have the six conductor cable.
Anyway, it's a good example of how functionally similar IBM keyboards were between product lines, in this case to the point that they can be swapped! Only one way, mind...