It looks like you need the daughter board (which based on your post you have this). There appears to be two connectors on the base (11C) board labeled J1 and J2 where the daughter board connects.

A quick look of merchants and I cannot find the daughter board separately.

It also looks like there might be two revisions of the board. One with the connectors and one without.

mpu11c (resized).jpg

I'm fairly sure you need the daughter board.

U9, U21 and U22 (and other associated parts of the circuitry) on the main board are required and these are not present on the 11C or 11C Rottendog replacement. The daughter board contains the additional components that were not populated on the 11C main board.

For 11B there's still some audio circuitry on the main board. There is a separation between sound and speech (I can't remember which board does what) but the responsibilities are separated between the main board and the audio board that you have identified as a separate board in the back box.

11C boards can only be used in 11C unless you populate the missing components between 11B and 11C.

I believe 11A/B boards can only be used in 11A/B/C unless you populate the missing components between 11 (nothing) and 11A/B.

11 (nothing) boards can be used in 11 (nothing)/A/B/C although the diagnostic LEDs are a 7-segment display and it blinks in a weird way.

In essence Williams removed more and more components with later and later generations of system 11.

Rottendog 11A/B board with daughter board should look like this.

MPU011A (resized).jpg