I have a pair of CMP purchased carbines and one cadet rifle, all of which both have an large brass insert riveted into the magazine effectively acting as a block, only allowing single loading. 

I have a copy of Poyer's book but can find no reference to this.

Hoping someone here knows what it is and why it may have been installed.   These are some of the guns that came back to CMP years ago from the various veterans groups when there was a Clinton administration order for that to happen >:(.

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