The AGC body is milled magnesium. It consists of two halves, called tray A and tray B. The logic, memory, interface and power supply modules plug into the trays on the insides, the parts that face each other when the AGC is assembled. The outsides, the outer faces, have wire wrap connections and an outer flat plate with gaskets.
With the plates in place on the two outer faces, the AGC is hermetically sealed and it is left pressurized with nitrogen to keep outside air from entering. Apparently the spacecraft environment has too much humidity, corrosive vapors and other contaminants for reliable operation, thus the AGC is sealed.
One side of tray B has chambers that allow the core rope memory modules to be slid in and removed without taking the rest of the AGC out of the spacecraft or opening it up. Software updates are delivered via new core rope modules (read only memory).
Changes can be frequent, especially when a problem is detected on one Apollo flight and changes are made to accommodate that. Since the pipeline processing Apollo missions was well overlapped, several spacecraft were in process at one time. They might be getting checkout in the Manned Spacecraft Operations Building (MSOB), they might be stacked partially or fully in the Vehicle Assembly Building (VAB), or they might be erect at the launch pad atop the LUT on the pad.
In all those cases, changes may need to be made and swapping without disassembly is a real timesaver. The block I version of the AGC, used only in the unmanned missions, had the ropes installed inside the trays, but an important block II change was to make rope module swaps faster and less disruptive.
Two connectors on the AGC allow connections outside the computer. The primary flight connection, A51, is a 360 pin connector that links the AGC to the rest of the spacecraft. Another connector, the 144 pin test connector is normally sealed with a special cover that loops certain signals between pins, but that cover is removed to hook the AGC up to ground support equipment for checkout.