When in doubt treat it as corrosive. However, you only need hot water. No soap or ammonia is necessary.
Kynoch made and still makes commercial hunting ammo too. They still load a 215 grain SP.
1931 dated ammo will be Mk VII(Accepted in 1910) loaded with smokeless powder using either a Boxer or Berdan primer. A 215 RN(Accepted in 1904) military ball cartridge is a Mk VI and is Berdan primed with smokeless.
Arms Corporation of the Philippines currently uses the 'ELEY' head stamp too. 'ELEY London' is a pretty safe bet that it's not from the PI though. snicker. Especially as ELEY merged with Kynoch in 1926.