You be interested in doing some research of posts made during the past year on pistol type detectors in this forum. From what I have read, I might speculate that there is indeed a coil in the pistol that does not use a shield. From what I recall, an unshielded IB coil has some unusual distance properties if you are using it in an area far away from electrical interference. The use of IR has been explained as a method to augment the extremely weak signals received by means of imparting heat energy to the target area. But even without the IR, these unshielded coils were said to pick up weak signals at much greater distance providing the sensing circuits were very finely tuned in a different manner than a conventional detector is tuned. I also noted the early IB method was said to work, but as other standard metal detector technologies were developed, it was found they also worked as well or surpassed the performance of the IB circuits. I recall a large frequency range could be used, but a certain range worked better (I assume the circuit you are investigating uses the best frequency found by the experimenters).
Best wishes,
J_P
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