Details of Trigger Bits

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Details of Trigger Bits

08 August 2015 "Practical Aspects of Directional Drilling" presented in 1946 mentions the use of a "Trigger Bit" through which a survey barrel could be run and allowed to protrude enough below the bit to "get a directional picture".
The reference for this paper is; API-46-009 "Practical Aspects of Directional Drilling" D.K. Weaver, 1946.

Does anyone have any other information on these Trigger Bits?
Old School for sure - 1946 really is the year and is not a typo.