My J&W Tolley had one hammer missing. Got a matched pair with the right throw, need to fit them. Is there any trick to getting the geometry of the square hole right? I have one of the old hammers but there is no easy way to trace that geometry. So far I have just drilled a hole a tad under the correct diameter. Now I need to square the holes. And one more question, should one anneal the hammers to make it easier to file them? Right now they are pretty tough.

Of course center the hammer where it need to be and drill the hole for the hammer mounting screw. Of course you have not filed the square yet. Heat and place a nice coat of solder on the back side of the hammer where the sqaure will mate. Place the hammer on the square (tumbler) and carefully snug down the screw exactly where you want the hammer to be. Then lightly strike the hammer with small hammer on a socket wrench that fits over the head of the screw. We don't want to injure those threads on the screw and tumbler. The solder on the back side of the hammer should show a perfect fit of the tumbler. Since you have already drilled the hole near to the tumbler size, you may want to fill it all with solder and drill that small hole first, then do what I suggested. Carefully file the hammer to the correct fit and don't forget to give the sqaure hole a taper, so it tightens as it is placed on the tumbler. Of course when finished, heat it all and clean off the solder. Yes, I would anneal the hammers and caseharden them after I am content with my fit. I hope, I didn't confuse you....

Thanks, clever solution!

Got one on this morning by marking the diagonal position, want to make sure the second one is identically positioned. Will use the solder trick for sure.