This spacer allows the use of shorter reloads in longer Pro29 casing.
You can use, for example, a P29-3G motor with reloads made for P29-1G and P29-2G motors simply adding two of this spacers inside the motor, in front of the reload. 

With two of these spacers you can only have a P29-3G and a P29-6G motors to use any Pro29 reload.