I came into a supply of boxes while cleaning out a relative's estate. The boxes say:
Controlled Magnetic Transducer
Shure Brothers, Inc.
Model No. 99K561 Q1
Can you tell me anything about what these are and how they were used?
The 99K561 transducer was used in the model TH41Y - a telephone-type handset made for the U.S. Government. The handset featured a "noise-cancelling" mic design, and a transistorized preamplifier.
The 99K561 was discontinued in 1990.