Most of the 9 inch housings you are going to run into will have a larger hump in the middle backside, and an overall more round appearance as compared to the oval look of the 8 inch. If it just the center section, the 9 inch you will most likely see will have one single vertical rib on front of case (unless you find a War or N case-both rather rare in overall numbers out there). The most abundant 8 inch case will be the C7OW- it will have two short vertical ribs, or the earlier 8 inch case- which has two horizontal ribs across front of case. See my web page for photos - link below.