The 25db Stratocaster Mid Boost Kit is an official Fender mod to introduce active circuitry (namely a controllable mid boost) to almost any Stratocaster (it comes pre-installed on the Eric Clapton signature strat but with this kit can be used with virtually any strat pickups).
The 25db Mid Boost circuit was first used on Fender's 'Elite Stratocaster' in the 80s. The Elite Strat was only in production for 2 years but the Mid Boost circuit found a fan in modern legend blues guitarist Eric Clapton who insisted Fender include the Mid Boost circuit in his signature guitar. Now various models of Eric Clapton Strat have come and gone but the Mid Boost circuit has always stayed. He loved it for focus and tightness it gave his tone (he often described it as 'compression' but many others like to use it for its p90-esque beefy single coil sound)
Here's what Fender say about the Stratocaster Mid Boost Kit
The Stratocaster Mid Boost Kit includes all the parts needed to upgrade your guitar with the Mid Boost Preamp used on the Eric Clapton and Buddy Guy Strats. The system replaces one of the original tone controls with a variable “Midrange Boost” control. The Mid Boost knob adjusts gain from 0 dB (no boost) to 25 dB (for killer mids). Great for clean and distortion tones. The TBX tone control is neutral in the middle position, cuts treble in one direction and cuts bass in the other direction.