Years gone past before the invention of meat thermometers, you tested your cooked ham for readiness by pulling at the mustard bone and if this came out relatively easily, then you knew your ham was fully cooked.
The bone would be the shape of a small mustard spoon as we know them today with a small bowl in the opposite end, which happens to be thicker. Here we would use the mustard bone to apply the mustard to our ham.
The same bone by holding it with our thumb in the bowl end and having the narrow end sharpened to a point, we could then use this bone to obtain a sniff test from our air-dried hams to ensure there is no spoilage in the centre. We just simply push in our mustard bone into our ham near to the centre as we can and withdraw the mustard bone and sniff. If there is any spoilage, you will notice it immediately. Ham spoilage attaches itself to bone far more easily than it could to a metal skewer for example.