2.5mm option is designed for B41, B42, B43, T24 series with Regualr PTT.
3.5mm option only works with T24 with Large Junction Box PTT and T25 series.
ARC Radio Accessories Finger Remote PTT
WARNING: This product may contain chemicals known to the State of California to cause cancer and birth defects or other reproductive harm. For more information, go to www.P65Warnings.ca.gov