Here's an interesting story on the history of the K240's, perhaps written by an AKG representative:
http://www.akg.com/forum/index.php/topic,1749.5/wap2.html
here's the text of that post
"audioburg:
Hi, finally here you are:
The first K240 was introduced in 1975. From its design it looked similar to the K240 models we know from current production. Only the metal rings over the ear cups were of silver colored polished aluminum and the inner headband was perforated. Like in the recent version K240 Monitor a 32mm driver was used but it was surrounded by 6 passive radiators. Each radiator consisted of a membrane attached to a perforated disc covered with carefully selected acoustical friction material. Conventional circum-aural headphones often have a resonance peak in the upper bass region followed by a more or less significant bass roll off. The result: an unnaturally sounding boomy bass. The resonant frequency of the passive radiators in connection with the relatively high friction of the damping material behind these membranes effectively influences this irregularity in the frequency range.
In the region of the bass boost the membranes become transparent to sound waves and thus reduce the air pressure in the coupling cavity between capsule and ears. In that way the bass boost is leveled out. Below this boost-frequency the membranes block the sound waves and the roll-off is compensated. This acoustic principle was invented by Dr. Goerike, co-founder of AKG.
The first K240's had a four core cable and were available in two connector versions. One had the standard 6,3 mm stereo jack while the other carried a 5 pin connector according to the German DIN standard. This plug was known as Wuerfelstecker (Wuerfel = dice) because the 5 pins were arranged like the dots of a dice. The advantage was that the connector could be plugged into an amp or TV set in two directions. One caused the internal speaker to be turned off, while in a 180-degrees turned position both speaker and headphones were on. For this trick it was necessary to have all four terminals of the headphone drivers connected to the plug.
In France the K240 was named K242 for marketing reasons and during the first years of it's production.
Headphones with the same working principle as the K240 but with individual design were built by AKG for Philips (N6330), Saba (H200) and Uher (W774 and W775).
Although the first series of the K240 were a great success, there were certain discussions among the experts at AKG if the same effect as achieved by the passive radiators could be reached or even surpassed by a different design possibly less critical in production. This brought the introduction of the K241 where instead of the passive radiators only various sound ports covered by acoustically resistive material were used. The dynamic drivers were surrounded by apertures closed by carefully selected friction materials with various values of perviousness related to the acoustic impedances of the outer ear. The K241 was introduced in 1979.
Soon after the introduction of the K241 it's principle of the variable acoustic resistors – patented by AKG – was applied to the K240 as well and replaced the passive membranes.
In 1984 AKG introduced the K240DF, acoustically equalized to the diffuse sound field. The K240DF refers to such a sound field present in a reverberation chamber using third-octave filtered pink noise and measured in the close vicinity of the ear drum with a measuring probe. The work of the IRT - Institut für Rundfunktechnik (Institute for Broadcast Technology) in Munich proposed a standard for headphone-monitoring calling for the diffuse field equalization of monitoring headphones coming closest to loudspeaker monitoring, because there existed no current IEC or CCIR standard for such a system. Especially at times, when program material is exchanged between stations internationally and the monitoring facilities may vary it looks extremely sensible to exclude the varying acoustical parameters of the different monitoring rooms and refer to standardized headphone monitoring when comparing program material.
The K240 Monitor as made from 1984 to 2006 was based on the K240 without passive membranes but with the ear related resistive sound ports. It's earshells were decorated with gold colored metal parts and a three core cable with universal 6,3/3,5 mm (1/4" and 1/8") plug was attached.
In 1997 when AKG celebrated it's 50th anniversary the "K240 Jubilee" in limited edition was made. Acoustically a K240M, it had different nameplates and came with a booklet with a short history of AKG.
In 2002 the 'Studio' Series of headphones was launched. A newly developed capsule with low impedance makes these products especially suitable for portable equipment. The cable is easily detachable. The first series had a minijack connection while from 2003 on a miniXLR-connection between earshell and cable is used. In the same year the closed version of the K240 Studio called K 271 was released. "
