The one on the left has a GMCS bullet up front and the one on the right a cupro nickel clad jacket that does not react to a magnet. The two headstamps are very much alike, so they loaded two different bullets in 1943 cases. The cartridges weigh 383 grains (GMCS) and 388 grains (CN). I am still searching for books on .303 hence my question.
edit: added picture