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
SHIMANO TOURNEY Right Shift Lever 6-speed | SHIMANO BIKE-EU
Bike Twist Right Shifter 6/Speed Shimano Revo. bike part, bicycle parts. - Walmart.com