Well this plane could have been one of those that was imported from another, older sim. (Ex: 2002 -> 2004 , 2004  -> FSX)

Each sim had its limits, and most model's came close to the maximum textures allowed by the sim at the time. Among other things, sometimes the plane you have is the result from hours upon hours of finding "photo accurate" textures of parts of the plane. Store bought digital cameras weren't as cheep as they were now!

Generally speaking though, you could try to get in contact with the creator of the airplane if they left an email for you to use. Before you try anything else, do that first.