pscartozzi,
Those are pistol ports. The cable is used to secure the removable plug from inside the fighting compartment. For close in defense the plug can be pushed out, so that it hangs on it's cable, then a pistol or hand held weapon can be used to fend off any close in infantry attack. The cable or chain can then be used to pull the plug back into a closed position. The Germans used a similar design. Here's a good example of a deloyed plug on a Sturmtiger.

Hope this helps a little!

Craig