Feel cover on the back side of the pump. I have read that the rear cover on the 17* pumps has a raised bump rather than being just a flat plate held in with a circlip, although some 17* pumps have a flat rear cover. However, no 15* pumps have the raised rear cover. In my experience, this has proven true. I have a 1997 F-250 and the cover on it is flat, and I just bought a pump off of a 2001 that has the bump on the rear cover. So, if the rear cover feels flat all the way across the diameter, you probably have an older 15* pump although it is possible it is a 17* pump. If it is not flat but has a raised surface, it is a 17* pump. Hope this helps. Has anyone else heard of this method of differentiating between pumps?