The AP-74 is totally optional. You hook the servos up to the EFIS via DSAB (2 wires). Once you hook up one or two servos, the EFIS starts displaying an "AP" menu, which lets you control the AP. This menu replaces the "BUGS" menu, and then BUGS are inside that menu.
You can literally hook one $750 servo to a D10A, D100, or D180 and get full roll or altitude control. Nothing else is needed, except maybe an install kit.
The AP74 gives you hard, external buttons for the AP functions. These are the same functions that are inside the EFIS menu, but they're right there on the panel. The one thing that AP74 adds is the ability to pre-select what mode will be active when you engage the AP. So you can hit "NAV" before you hit "engage" and it will come on in NAV mode.
You also get a knob to set bugs, voice alerts, and auto-dimming with the AP74. The connector on the AP74 is a male D25. The pinout is the same as the HS34's main connector, but the only pins used are power, ground, DSAB, and audio. You can also hook your disconnect buttons on your sticks to the AP74.
Each servo has a wire on it for a disconnect button, so you still get a hard disconnect on your stick even without the AP74. This button can be used to disengage the AP, and can be used for control wheel steering, or you can hold it for a few seconds to engage the AP.
The AP74 will be available horizontal the day it ships.
Let us know what you want to know!