DEC MKA11 and/or M8261/M8163

I don't understand the role of the M8162 "Port MUX A module" and M8163 "Port MUX B module" that the field-guide claims to be part of the MK11. They aren't part of the MK11 as far as any documentation that I've read regarding it. At least the M8162 has a pair of 20ma-style SLU connectors on it, but no UARTs that I can see. Do these modules really belong to the MKA11, rather than the MK11? Or to something else entirely? Is the MKA11 (which I can't find documented online) a revised MK11? The field-guide seems to imply this given its entry for the M8164 which basically reads like this: M8158 MK11 U Address buffer module M8159 MK11 U Data buffer module M8160 MK11 U Control A module M8161 MK11 U Control B module M8162 MK11 U Port MUX A module M8163 MK11 U Port MUX B module M8164 MK11/MKA11 U Data buffer module (Replaces M8159) It seems to me that something is incorrect in the field-guide, but none of my researches have identified where the error is and what the right answer(s) might be. Anyone here have any experience/knowledge with/of the MKA11 and/or the M8261/M8163? Thank you, paul