Raimey,
I couldn't find anything that would absolutely require reproof either. I noted that the reproof was performed under the 1939 proof law, which was the first time it was required that the cartridge that was used in the gun was required to be shown by the name it was commonly called. My thought was that it may have been voluntarily submitted for proof to have the nominal chambering shown. Axel reported, some years ago, that rechambering 9.3x72R and 8.15x46R only to normalize the chambering did not require reproof. This was because the normalized cartridges were loaded to the same pressure as the old.
Mike