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Page 24 of the Glock Armorers Manual:
Locking Block Pin Removal
THE LOCKING BLOCK PIN SHOULD BE THE FIRST PIN REMOVED (FIG.29), AND MUST BE THE FIRST PIN PUT IN OVER THE TRIGGER DURING REASSEMBLY.
The locking block pin is the first pin out during assembly and the first pin in during reaaembly.
CAUTION: If you install the locking block pin after inserting the slide stop lever, you will bend and damage the slide stop lever spring.
The slide stop lever spring bears on the locking block pin.