Thats easy, the airplane originally was equipped from the factory with AN type gyros. The heading indicator/directional gyro/directional indicator, whatever you want to call it (I call it a DG) was a type with a face that resembled the compass, with a horizontal drum, instead of the vertical card of the modern DG. It was not the full face type. The original AN type DG has a smaller than normal face with a caging and reset know well below the face. People will open up the cutout in the panel and install a full face, standard size 3-1/8" DG in its place. This leaves the hole in the panel below the face and if you look at the plastic, the hole in it around the face has been opened up to this size.
The AN horizon gyro/attitude indicator is much larger than a standard 3-1/8" insturment and requires a filler plate to mount a modern attitude gyro in the place of the old AN instrument.
Look in your parts book, you will see what I mean, regarding the appearance of the old gyros.
