The same seller who sold me the turquoise 591 has an aqua 554 on ebay now: ebay link (http://www.ebay.com/itm/180831706442). ( dead link 03-31-21 )
He told me that "123 A 55412 lr 30m 10 67" is printed on the back of the phone. The "12" probably means aqua. I offered to post about it here.
Larry