Victoria 1855 Half-Sovereign, unpublished overdate, 5 struck over 1

Victoria (1837-1901), gold Half Sovereign, 1855, second 5 struck over or possibly 3, first young head left, type A1, date below, legend and toothed border surrounding both sides, VICTORIA DEI GRATIA, toothed border around rim both sides, rev. crowned quartered shield of arms in frame, BRITANNIARUM REX FID: DEF:, weight 3.99g (Bull EGC 1251; MCE 577; Marsh 429; S.3859). Toned, with light surface marks, almost extremely fine, the overstruck date showing the top serif of probably a 1 sticking out the top left of the second 5, a curved flaw perhaps below rather than the curve of a 3, a trace below that of a part of lower left foot of a 1, all to the left of the 5, an intriguing overstrike that is currently unpublished.