RCA SK-39A

Our example appears to be a revised version - it is stamped MI-12040B, whereas the data sheet gives it model number MI-12039A

From the RCA data sheet:

The Type SK-39A Aerodynamic Microphone has excellent response for close talking purposes, its light weight and small size make it ideal for remote pickup and mobile use. It is used for paging and announcing in areas of high noise level because its rising high frequency characteristic gives excellent intelligibility. This feature is especially useful in home recording and amateur applications. Another application for which this unit is especially suited, is for use of an individual soloist, where a second microphone, usually a velocity type, is used to pick up the musical accompaniment. Either a floor stand or a desk stand may be used as a mounting or it may be fitted with a handle for use in sports announce work.

The SK-39A has been designed and constructed for dependable performance and rugged service. It is relatively insensitive to mechanical shock and wind disturbances and will withstand nominal exposure to moisture or rain because of its plastic diaphragm.

Taken from the 1959 RCA Catalogue:

• Directional Characteristics: Semi-Directional. Non-directional for frequencies below 2,000 Hz; becoming less sensitive at the rear and sides as the frequency increases above 2,000 Hz
• Frequency Response: 70 to 10.000 Hz (see curve)
• Output Impedance: 250 Ohms
• Output Level (1000 Hz): - 54 dBm, Effective (10 dynes/cm*)
• Hum Pickup (0.001 gauss, 60 Hz): - 105 dBm
• Weight: 1 lb. (0.45 kg.) less cable
• Modern streamlined appearance
• Excellent for close talking application
• May be used outdoors - minimum response from wind and unaffected by temperature or humidity 
• Alnico V magnet - high sensitivity with light weight

A description of the RCA SK-39A at the Coutant Website.

Cole Suddarth's opinion: These are great microphones for effects when looking for a defined vintage coloring. I’d recommend these on almost any source where texture is needed.