A boundary layer microphone from Dutch company HopProd, actually more of a system since you bought the power supply, the microphone and other bits and pieces separately. Originally constructed as an alternative to Crown International PZM microphones and does indeed follow that pattern rather than that of a classic boundary layer microphone and is the only such we recall seeing on the market that was not licensed from Crown, that the PZM patents expired in 1999 seems not to have changed this.
The manufacturer boasted that these provided a better sound at a much lower price than what Crown was charging, in particular the top end was considered better. However this increased performance appears to have been gained by overdriving the back electret capsule with a 9v current rather than 1 to 4 one that is needed, this is a common trick in the DIY community to get better sound and signal to noise characteristics out of budget back electret capsules, the problem being that you lose quite a bit of the dynamic range and that only Sony electret capsules appear to survive this unaffected in the long term and really gain from it as well, the capsules used in this microphone look more like Panasonic ones which benefit more from disabling the internal FET and driving it with an outside source, so keep in mind that second hand examples of this mic might need capsule replacements, but these are easy and cheap to find.Note that all the original prices listed below are in the now defunct currency of Dutch Guilders, to give you a coarse example of the prices from memory a stereo set (Standard set type HPM 5797.) with a preamp/power supply was somewhere around 200+ Euros/ 250+ USD in ca. 2000. This microphone appears to have been on sale from about 1998 to 2003 or so.
HopProd Recording microphone HPM5790
Frequency-range: 30-20,000 Hz (2db).
Sensitivity: 0,6 mV/ubar/1 kHz.
Impedance: 2kOhm, RL-2k2.
S/R-relation: larger than 44 db.
Power supply: 9V off power supply unit.
Power consumption: 2mA per microphone.
RRP: NLG 140 in 2001
Basic module (stereo) type HPM 5791
Module 2 (mono)type HPM 5792
Module 3 with built-in amplifier (stereo) type HPM 5793.
Module 4 with built-in amplifier (mono) type HPM 5794.
Module 5 with built-in amplifier and balanced output (mono) type HPM 5795.
Module 6 with built-in amplifier, balanced output (mono) and phantom power type HPM 5796.
Standard set type HPM 5797.
The microphone itself was made in Japan, Sony did at the time do OEM microphone manufacturing for Tandy Radio Shack so it is possible that the company made this mic, but it is also possible that another sub-contractor made the unit using Sony electronics. The original model was replaced with the Realistic 33-1090A at some point in the mid 80's, it is identical to the original model except that the extrusion/housing that holds the cable assembly and microphone on top of the plate is smaller than on the original model, but there were complaints that the unit performed better facing "down" than up, i.e. that the original housing interfered too much with sound coming from below.
The mic was upgraded again in late 1992 with the last model the Realistic 33-1090B that was sold until late 1997, this time 2 really simple mods were done to it, it has a hole on the plate to simplify fastening it to a wall or such and a black foam pad was placed around the mic at the request of those that used the unit outdoors, most people used the unit indoors fastened to a wall and removed the foam pad altogether. Note that the actual catalogue number remained the same even though the product number got an A or a B added to its end, this is the norm for Radio Shack, they often switch the actual products in behind each catalogue number.
RRP : 33-1090 USD 39.95 in 1984, 33-1090A USD 44.95 in 1988, USD 49.95 in 1990, 33-1090B 59.99 in 1997.
Manufacturers technical specifications :
Freqency response: 20 - 18,000 Hz
Impedance: 600 ohms
Sensitivity: -74 dB (ref.: 0 dB=1v/mbar, 1k)
Signal to noise ratio: >40dB @ 1 kHz
Battery Type: AA DC 1.5V Dry Cell
Battery life: Approx 2000 hours
Current consumption: 500mA Max.
Output: 6.3mm (1/4") jack terminating a 550 cm cable (18 feet).