Not sure. In theory, you should be able to set a path lest you have 2 prototypes with the same name. However, same-named prototypes are not a good idea. If you use unique names for prototypes, then you’ll never need to worry about needing to use a path to set a prototype.
Tip: use the built-in
/Prototypes container for your prototypes. Even if you don’t want to use the built-in prototypes, just add one to let it make the container and then delete the unwanted prototype. In the long run it will save you some confusion. Also, the container Tinderbox sets an action so that all new children added to it are set as being prototypes, etc.