I am thinking they were mostly made of some very soft material, mesh or some type of rubber. The sound effects brought the Stooges to life. If you have seen them on a live show, there are either no sounds or the sound effects were done by a tech or musician, and not exactly on cue. Its very different than watching a short. The sound of hitting someone on the head with a broom handle would not actually have the sound of two iron skillets clanging together, for example. When you punch someone in the stomach, you shouldn't hear a bass drum either
I can imagine that the heads of the hammers that they hit each other with were of rubber, but the crowbars and broom handles were clearly of a more rigid material.
Certainly the sound effects are crucial to the comedy. One of my favorite Stooge gestures is the head-conk (especially, of course, when preceded by the ritual brandishing of a fist and utterance of, "See that?"
), but it would not be funny without the sound that they used for the impact.
I started the thread in the hope that somebody knew of an interview or an article somewhere that gave the inside story on these props. If not, then we are just left to speculate. I'm surprised that nobody ever asked the Stooges about it while they were alive.