This is the 15.5mm inner diameter type (approx 1998 on)
A common fault with the iB5 is jumping out of reverse gear, wear on this gear is often the cause.
What problem does this part solve ?
Replacing this gear (and if required with the 1st/2nd synchro hub) the problem may of your gearbox sliding out of reverse be fixed providing the lay gear is not worn badly on its reverse gear section
The gears are new unused items removed from an unused gearbox