Where are the air filters typically located on an older model horizontal luxaire gas furnace?

Most are located at the point where return air vent joins the furnace,either inside or outside of furnace
Sometimes they are inside the return air vent in your house,there are several options for a filter ,but must be blocking the return air vent somewhere

Ok. I have three return air filters in my house that I maintain regularly. I was looking for one in or near the furnace in the attic. I guess the inside filters are all I have. Thanks.

I have a

Sorry, I hit the enter button. Here's a link to the Luxaire (http://www.luxaire.com/CatalogPageLuxaire.asp?Var1=Residential&id=245) site. If you go to the bottom you'll see tech questions and manuals, etc.