My daughter's Trek came with a 6-speed Shimano Revoshift SIS Index. It's a new bike. She can shift to higher gears easily but does not have the grip strength to shift back
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Mountain Bike 22.2mm Short Grip For SHIMANO SL-RS25 RS35 PVC EIEIO REVOSHIFT Shift Lever Grips 70/75mm Bicycle Parts
Shimano Tourney SL-RS35 Revoshift Grip Bike Twist Shifter Lever 3 * 6s 3 * 7s 18S 21s Bicycle Comb With Grips Bicycle Spare Parts (Color : 3x7 Speed a pair): Buy Online
SHIMANO - 7 speed Twist Grip Shifter (Right Shifter), Revoshift Sis In – Bike Blvd
Shimano RevoShift SL-RS31 3 x 7 Speed Bike Grip Shifter Set + Grips NEW - Walmart.com
Shimano Revoshift Grips - Grips - AliExpress
Replacing Shimano Revoshift With SRAM Grip Shift Shifters - YouTube