This is the DiMarzio model that Mark Knopfler used to have in his red Fender Strat in the late 70ies. It has a hotter and fatter sound (FS stands for fat Strat).
From autumn 1978 on, he had it in the neck position but I have some good reason (
see this blog post
) to believe it was in the middle position before.
The sound in the 1 & 2 position of the 5-way is really different with a DiMarzio in the middle, compared to two Fender stock pickups: less nasal, warm bottom and treble that cuts better through the mix -->
Sultans of Swing (first album version)
The FS-1 comes with his own white or black plastic cap but when being careful your existing pickup cap should fit. I deliver the version with white cap.
!!! Note: If you want all sound capabilities of the FS-1 in the neck or in the middle position, plus the normal vintage sound, you can go for the
VFS-1 pickup (Vintage Fat Strat)
which is a tapped pickup designed to have both sounds switchable !!! 
