Now that I'm home, I've checked and found that the interior dimensions are actually 3.5" x 3.5" x 9.5", for a total volume of 116.375 cubic inches of lead, which weights ~47 lbs. Throw in the weight of the steel, and that hammer head is going to be upwards of 50 lbs.
This raises an interesting question. The anvil and stand weigh about 300 lbs, which gives a hammer-to-anvil ratio of about 1:6. If I'm only planning to use this for punching, chiseling, and the like, is that ratio going to be an issue? Or would that only be a problem if I were planning to be drawing out, fullering, and swaging heavy stock?