This is to answer the mystery of the green M1 carbine Receivers. The method I used to discover this was the Delta XRF Analyzer which used resonant harmonics to determine chemical and elemental makeup of objects. In this case, the analyzer was used on a collection of M1 carbine Stubs that Ed Senior had given to a Mr. Hill. These stubs has the distinctive Green that you find on many military rifles. and the answer is..........
Copper........ Most likely a cupric oxide mixture in the Park or Bluing process. Attached is a link of me using the machine to test the stubs. I also used this to test a 1919 top plate that had the same greenish color and the results came back the same. I hope this information helps in terms of how firearms were refinished. The link is below. This machine costed $800.00 to rent for this project.
You will need to pause the video on the readings but you will be able to see that CU is definetely in the finish. I will try to find the graph readout a little later.