there are 2 different models included. The smaller one is not as resistant to belt slip as the larger one. The larger one is strong enough to hold the belt under as much tension as my printer is capable of putting on the belt so it will not slip. 

NOTE: as shown in the picture there is a notch in both sides of the larger end stop for the print head rollers. The first version of this file did not have the notches, and as a result the lower wheels bound up and caused flat spots to form. This should be resolved but verify there is room for the wheels to turn before use. 

the smaller belt stop may require you to tweak print settings for tolerance, the belt must fit VERY tight in the insert, and then the band that locks it together must be a right interference fit with the insert or it will slip under light load. For this reason the larger end stop is recommended for most situations unless space is an issue.