March 27, 2007
Unfortunately, I think that's what you have.
'5560' is the basic part number for 'Spring' in the Ford parts catalog. -AA is the revision.
'E5SC' is the parts prefix for a 1985 "E5" spring. "SC" generally indicates what car line ("S, which isn't "Mustang") ... not sure what it was originally used in. And "C" indicates what group designed the part. (Guessing "C" is 'car', but I've seen a variety of letters in this position).
I didn't originally guess this as a Ford part number because you posted it like you see it on the tag, instead of all on one line like you'd normally write it: E5SC-5560-AA.
A little googling on the part number seems to show that it was used in '85-'94 Mustangs, so I bet it's the original spring, cut or relaxed with heat.