The firm’s calibration bench, its counter-surveillance workshop and its paper records, which have filled two floors of 409 Madison Avenue since 1971, have moved to Building C, 1200 Meadowlands Parkway, Secaucus, New Jersey.

The records include every engagement letter, survey and placement the firm has made since 1962. They are kept in fireproof cabinets, in a room with no windows, and are seen by nobody but the firm’s archivist and, on request, the client concerned.

Clients need do nothing. The New York office remains at 409 Madison Avenue, and its telephone numbers are unchanged.