If you download and enlarge this image a bit, what seems to be the lower left corner of a 2 appears to be extending outside the six at around 7:30, at the place you would expect by extending the arc visible within the loop of the 6, with the trace of the horizontal extending left to right, and the upturned tail of the two very close to the lower outside right of the 6.  Similarly the outside of the upper curve of a 2 appears outside the upper arm of the 6 at around 11:00.  To me, at any rate.  If the underdate IS a 6 or an inverted 6 or a 9, it must have been from a different set of numerals as it appears to be much more narrow than the overdate 6.  I can find no mention of an 1861/21 half penny, though.  I hoped someone with access to better books than I might find information on this die.