knife-back

Moisten skin and remove visible phosphorus (preferably under water) with squared object (knife-back etc.) or tweezers.

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Out came the 'hardware' with that jarring cr-r-r-rick the blade makes when the notched knife-back catches in the spring, but Caper jumped between them, and they put off stabbing one another -- until the next saint's day.

The Continental Monthly, Vol. 1, No. 3, March, 1862 Various

I stood trembling almost to see her cross the knife-back of that ledge.

The Seaboard Parish, Complete George MacDonald 1864