I have recollection of the power mace in second edition being described as a precurser to the thunder hammer. I think this was in 2nd ed codex chaos in the wargear section but i can't find my copy to double check. i might not even have the book any more.
My point is that i believe it was included because no chaos models had thunder hammers because they had been equipped with power maces during the heresy.
I suspect there is probabley a marine armed with a thunder hammer somewhere in one of the more recent HH books but i cna't think of one at the moment.
you could probabley use power maces to rep thunder hammers as they were definately around at the time.