I saw my first example, a 1930's vintage FW Field Grade ejector gun two plus years ago (gun was written up in the LCSCA newsletter) and have seen several examples since; both FW and R frames. All milling cuts appear to be factory work, although I have never seen any factory literature referencing this feature. As the location and numbers of milling cuts/holes often vary, I suspect this work was done in order that the factory meet a special request as to weight, gun balance point, or both. As to the Houchins book, that book was already in print when we discovered the first example; as John was planning to update his book had he lived, I suspect frame lightening would have been on the list of topics to be covered in his revised edition.