The Worn SFIC Key and the Unreconciled Key Schedule
During a burglary investigation at a public high school, a key is recovered from the jacket pocket of a person stopped two blocks away. It is a 7-pin small-format interchangeable core (SFIC) key of a common Best-style system. The bow is stamped 'C 42317' and 'DO NOT DUPLICATE'. The blade shows heavy wear: rounded tip, polished flanks, and slight rounding of the cut roots. The district's facility records list four buildings on one master-key system with operating keys, master keys, and control keys, but the records are a spreadsheet last reconciled before a 2019 rekey. The district wants to know whether this key opens the building where the break occurred. The prosecutor wants an opinion that it is a 'master key' because no room number is stamped on the bow. The key has not been tested in any lock. Evidence intake records a single key, no envelope, no seal.
1. Design the examination sequence to answer whether this key operates the burglarized building, and state where the sequence stops being probative.
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Sequence
- Intake documentation. Photograph both faces and the blade with a scale; record wear, stampings, and any trace material before handling. Package separately, sealed.
- Keyway identification. Compare the blade profile to the system's keyway chart or a known blank. Keyway is class evidence only; it establishes the family, not the building.
- Decoding. Measure each of the seven cuts with a key gauge or optical comparator against the manufacturer's depth specification for that keyway, recording increments and tolerance. Heavy wear at the tip and roots widens the uncertainty, so report each depth with a range, not a single value.
- Controlled functional test. First test in a laboratory lock or core of the same keyway, then in the actual building cores, with video and a witness. Observe whether the plug rotates as an operating key or whether the core releases as a control key.
- Records comparison. Compare decoded bitting to the bitting list or pinning charts for the four buildings, not to the bow stamp.
Where it stops being probative
A functional test in one door proves the key operates that core in its current condition; it does not prove which door was used, when, or by whom. In a master-keyed system, one key may operate many openings, and a key that operates a lock need not be the key used in the offence. The unreconciled schedule limits any statement about access level.
2. Explain how you would determine whether this is an operating key, a master key, or a control key, and what the 'C 42317' stamp does and does not tell you.
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Function is determined by behaviour and bitting, not by the bow.
- Operating key: rotates the plug and throws the bolt; cuts align the pin stacks at the operating shear line.
- Master key: also operates, but its bitting sits at a higher level in the pinning hierarchy and operates more than one change key's set of cores. This can only be established against the system's pinning charts and by testing across multiple cores, not from a single lock.
- Control key: cut to the control shear line. When inserted and rotated it releases the core retainer so the SFIC can be pulled, rather than operating the lock. The functional test is unmistakable: the core comes out.
The stamp
A 'C' on the bow is a common convention used by some systems and locksmiths to mark control keys, but it is a convention, not a specification, and anyone can stamp a bow. '42317' may be a blind code, a dealer sequence number, a blank-lot number, or an internal reference; a blind code is meaningless without the system's code book. The stamp is an investigative lead to be checked against records, not a finding. Only the bitting and the functional test carry weight.
3. The prosecutor wants the report to describe the key as a 'master key' because no room number is stamped. How do you respond, and what wording do you use?
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Response
Explain that the absence of a room number is expected on a master key but does not establish master status: master keys are frequently unstamped, and so are unassigned blanks and keys from other systems. Master status is a relationship between the key's bitting and the pinning of multiple cores. Without the reconciled bitting list and test results across the four buildings, the statement cannot be supported. Offering it would be an opinion shaped by the conclusion the prosecutor wants, which is exactly what an expert must avoid.
Wording
The key is a 7-pin SFIC key of the [identified] keyway family. Decoded bitting is [values] with a tolerance of [range] due to wear. It operated the core tested at [location] in the condition received. The available key records are incomplete and not reconciled to the current pinning, so I cannot state whether the key is an operating key, a master key, or a control key within the district's hierarchy, nor which openings it operates. The bow stamp 'C 42317' is not determinative of function.
Also flag separately that the key's association with the stopped person and any use at the school are investigative questions, not lock examination findings.