The symbol in the prologue might refer to whatever metal Tan used to have Wax's shot kill Lessie, assuming it wasn't atium (which there isn't supposed to be any of at this point in time). And perhaps it simply is the number zero?
In terms of the compass in HoA, this is what the symbols seem to be:
North: Tin. "K"
North East: Pewter. "I"
East: Zinc. "D"
South East: Brass. "R"
South: Copper. "L"
South West: Bronze. "T"
West: Iron. "B"
North West: Steel. "P"