Just to expand a little on Reg's post, I was told by a reputable source that the Goldscan 2, which came out in the mid 1980's, was an influence on the development of later Minelab PI's. That being so, the use of 300uH coils for the GS2 may have been why Minelab subsequently used that figure.
As to the connectors, the original GS5 used Amphenol 3 pin connectors, which is what I have used on many other detectors. Being aware of what happened when the Infinium came out, where there was a big lack of choice in coil sizes, I did some tests on Minelab and Coiltek coils to see if they would run OK on the GS5 circuit. The inductance was obviously OK but the resistance was about a third that of my coils. This required the use of a current limiter so that the transmitter circuit was not overloaded and battery life shortened. Using this limiter, aftermarket coils worked fine, although some struggled to work at 10uS delay and required the pulse delay control to be turned back to 12 or 15uS. Being able to use Coiltek, Nuggetfinder and Minelab coils meant that a wide selection of coil sizes and shapes were instantly available without my having to increase my coil manufacturing facilities. The problem was that a different plug was used, and initially the simple answer was to make a short adapter lead with the appropriate connectors at each end. Having worked on military electronic equipment in the dim and distant past, I recalled that the most unreliable components of any electronic equipment are plugs, sockets and connectors and the fewer you had, the better. This was quickly proved true, and the decision was taken to use the same plug and socket on the GS5 that Minelab PI detectors use, and to use the same connections. This proved to be a good move, particularly because GS5's are sometimes purchased by SD/GP users who already have an armoury of coils. Why would they do this? Well, as Reg and Steve Hershbach point out yet again on another forum, different detectors have different characteristics and where a Minelab unit won't work, the GS5 or TDI often will, so it's another tool for a different situation. It was logical that Whites should also tap into this coil compatibility situation, although, having a large manufacturing facility, they may well bring out a variety of coil types and sizes themselves. Today, neither the TDI or the GS5C employ a current limiter as the transmitter design has been changed to accept a wide range of coil resistances without drawing excessive current on low resistance coils.
Eric.