Based off the limited production TIE Advanced x1, the Model-929 TIE Advanced x1 were discovered by the Darkest Night in the Unknown Regions in 2 BBY, aboard a derelict cargo vessel secretly carrying them. Despite the vessel having sustained heavy damage that seemingly killed all onboard due to sudden explosive decompression & ebullism, the cargo bay was sealed and intact, perfectly preserving its contents of 12 Model 929s, their replacement parts & fuel, and new fighter pilot suits (each suit labeled with IDMR; Imperial Department of Military Research).
Canon vs Legends
While the Model 929 featured First Order TIE Fighter Pilot jumpsuits & equipment, it should be noted that the First Order symbol is missing on the right shoulder pauldron and is replaced by the white bold letters of IDMR. Additionally, the SE-44C Blaster Pistol has been replaced with a DH-17 Blaster Pistol.
Reducing Weight, Increasing Speed
Despite the original TIE Advanced x1's increased weight, the Model 929 reduced that weight by removing the life support system and further reducing the weight of the exterior hull armor. These reductions increased speed with the already standard P-s5.6 twin ion engines to 110 MGLT, making them as fast as the TIE Interceptor.
The Model 929 housed a experimental Torplex Flight Computer & T-sj1A Targeting Computer, as well as an experimental subspace communications & sensor array (all made by Torplex). What Torplex is---or who controls it---is largely unknown, although considering everything is stamped with IDMR, it is a good bet who is behind the scenes.
These next generation hardware & software advancements funnel information directly to the new fighter pilot's helmet flip-down goggle displays, if so desired, increasing reaction times of the TIE Fighter Pilot. Additionally, the Model 929 lacks a life support system, reducing weight, from standard TIE Advanced x1s &.