the Model 721 was one of the predecessors of the Model 700's and many of the features of the 721 were retained in use on the 700's.
i would check with a knowledgeable gunsmith and see if the M700 extractors can be used on the M721. not for sure if they can, but wouldn't hurt to ask. because if then can be used, it might be easier to find one that way. just an idea.