It is hard to be certain about the scale of the under-strike, if any, due to the poor condition of the coin, but it appears to be of the right proportions to be a 6 or a 0. I attach a larger photo of the figure and a scale drawing of what I believe is visible. The outlined part is the actual 3, and the solid black is what may be the underdate. Like you say, this is very clear to the left, where a curved (and raised – so incuse on the die) limb peters away to little or nothing, but to the right it is much less obvious. I honestly can’t tell if my solid black part to the right is just part of the curve of the 3 and so purely conjecture and imagination on my part, or part of the curve of a 0 or a 6, mostly erased from the die. The little indent is interesting, though. How wide is your 3 at this point, compared to mine if you enlarge it to the same scale? That might help ascertain if there is anything there. Like you, I would plump for a 6 rather than a 0 from the fact that that digit occurs in the date already, and the fact that the 6 is pretty much the same as a 0 in size and shape but for its little tail. We really need to find a much better condition one, if possible! Is there anyone out there with one?