The part number 31099AA091 is for the external filter kit that Subaru installed when they recalled cars with the 4EAT transmission. This link shows the mounted filter:
http://www.subaru-svx.net/photos/files/How-To/14367.
After you download the file you'll need to add
.jpg onto the end of the filename.
The filter is mounted below the battery. The piece of metal at the bottom of the picture with the oblong hole in it is the front left side tie down point.
I hope this helps.
Sorry, I can't help you with the internal filter.
Dan