As others have been saying a white paint mark is what was done to mark the special leaf springs. If it is not visible then that is not out of the ordinary given where it is and the variation of how it was marked during typical production . I have seen original marks that appear to be have been made by brush and others by spray can . To elaborate more the Shelby rear leaf's are typically the same regardless of if GT350 or GT500 , fastback or convertible, AC or non AC equipped. If you are prepping the springs to be installed then I would first confirm that they are in fact Shelby springs. The paint mark is very hard to see once installed if at all. The special leaf springs on the other hand are VERY visible and can be easily identified as genuine Shelby or not. Also unlike other Mustang leaf springs you can not just switch the bottom plate like other Mustang leaf's and no one is the wiser. The Shelby leaf's are different shape (bottom) and lengths are different (next two leaf's)compare to regular Mustang. The only section that is the same as a regular Mustang is the section with the front and rear eyelets.
