From the 1968 Shure Brothers Microphone Catalog:
MODELS 405K CONTROLLED MAGNETIC MICROPHONES
Designed for clear, crisp, natural voice response of high intelligibility. The 405K Series Communication Hand Microphones are pressure actuated units using the Shure developed balanced-armature Controlled Magnetic Cartridge.
Model 405K-High impedance (14,000 ohms).
Output level, -14 db (0 db=1 volt per 100 microbars).
Direct replacement for General Electric models
EM10A1, EM10A2, EM10B2, EM10D, EM10E, EM25A, EM25E and
for use with “Progress Line” mobile radio equipment.