Could you pleasae post a picture of the clip here? It could be one of a few different items and a picture would clear up the ambiguity. One thought I had was that the "G.G. Greene" might be a subcontractor who made only clips, as opposed to the weapons.
What you have might fit one of the British rifles as they had 10-round detachable clips, such as the well-known "SMLE." The rounds that filled them were .303 rimmed cartridges, which caused the clip to be quite "wedge" shaped.
But rather than trying to guess what you might have, let's see a picture and be certain.
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