I apologize for the continued semantics in advance, but that's a
bridge model.
I couldn't find it on the main SD site, but you're right, the custom shop offers it (neck and middle too).
https://customshop.seymourduncan.com/pearly-gates-for-strat/
But...it ain't a
rail model like requested... And having first hand experience with those small polepieces on a Duncan SC sized humbucker, the volume drops when bending are real! So, unless it sounds be-awesome, probably best to steer clear.
I do wonder how it would compare to the larger version though, like with the SC sized Screamin' Demon.
I had the Little '59 for a while next to the Chopper in my Kramer SM-1 and between the two, the Chopper was ever so more versatile; it sounds great when split too. The Little '59 was nice, but the aforementioned volume drops were a turn-down.
That SM-1 was supposed to come with Cool Rails (n & m), but instead it came with 2 Hot Rails BRIDGE (!!!) models installed in neck and middle.
Too hot, too smooth; split was nice.
So I never got around experiencing the Cool Rails as intended in that guitar.