Look down from the magazine catch and you'll see a headed pin astride the block. Push this pin out from left to right as far as it'll go. )It is retained in the out and in position by a small spring loaded plunger.) Slide the mag catch forward and it will take the ejector block ut with it. You'll find the mag catch and the ejector block clipped together. But you've got your mag catch out.

Below the catch is the screw used to remove the locking shoulder.

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