I can do better than that, how about a quote direct from Mr Boeing!
A. Marker Beacon Receiver Module
(1) The active marker beacon module is located in the left VOR receiver
(AMM 34-51-00). The VOR receiver is located in the aft equipment
center on rack E6-1 or in the main electronic equipment center on
rack E3-2. The module in the right VOR receiver is inhibited.
I'll take a look again next time i'm at work and see if it sheds any light on the ACP's without the seperate marker selection.