Welcome. Tried to find that and source ereplacementparts.com listed it as unavailable and obsolete. Craftsman parts for routers are really difficult to find. The company had many overseas makers producing their tools, and because they kept changing makers, none of the companies produced many extra parts.
You could use a mocrometer to get the outside dimension of the shaft, but you also need the thread count per inch. Then you have to find some other collet with both those dimensions, and the right shape to fit into the spindle' angled inside cut. The nut may be easier to find, but the collet inside it MUST be an exact fit or the collet may not hold the bit properly.
Craftsman has fallen into the hands of a company that just exploits what used to be good brand names. It may be time to replace it.
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