B~T Sta-Tite Guide Rod in the M+P. 
It may seem redundant to offer a stainless captured guide rod to replace… a steel captured guide rod.
UNLESS… You’re a shooter who appreciates being able to use alternative spring weights in the M+P, on a guide rod whose design allows fast spring swaps.
This is particularly popular with competition shooters who use G17 springs in the M+P to achieve the recoil characteristics they prefer. Especially those who have transitioned from the Glock to the M+P. Those who are chasing the same recoil impulse that grew accustomed to by shooting their preferred spring weight in the Glock.
When you add a Factory S&W spring below, your rod will ship assembled as a unit, ready to drop in. Ordering an ISMI spring? You’ll want a toolkit in case you need to remove the spring to clip coils, re-install, test, repeat etc…