The way I view it, Coiltek's coil is "authorized" by Minelab to be used on the X-TERRA. On the other hand, NEL is not "authorized" by Minelab to be used on the X-TERRA. That means Coiltek worked with Minelab in designing their X-TERRA coil and NEL did not. It became obvious to me that NEL did not follow Minelab's engineering design when I saw they were selling one 7.5 kHz Tornado coil for the X-30 and 305 and a completely different 7.5 kHz Tornado coil for use on the X50, 505, 70 or 705. If NEL coils had been designed with Minelab's authorization and met Minelab's specifications, the same Tornado 7.5 kHz X-TERRA coil should work on any X-TERRA, just like the X-TERRA coils made by Minelab and Coiltek.
From a personal standpoint............... knowing that Minelab authorized Coiltek to build their X-TERRA coil, if something "went wrong" with my X-TERRA when using a Coiltek coil or one of Minelab's coils, my X-TERRA warranty would be in effect. If the NEL coil caused damage to my X-TERRA, the X-TERRA warranty would be void since NEL is selling a product that Minelab did not authorize. Performance aside, I prefer to do business with people who follow the law and do things right. Not some company who pirates someone else's licensed technology. JMHO HH Randy