The rapid reload head is compatible with string trimmer model numbers that end with F. Those include ST1500F and ST1500SF. They are not compatible with the old style bump head. The older versions of the string trimmers (all other model numbers) are not compatible with the rapid reload head. We suggest that if you do not wish to use the rapid reload head, purchase one of the older models of the string trimmer. Those will be phased out in the future and the only head available will be the rapid reload head. It should be noted that both heads are bump heads. The only difference is how the line is replaced within. The rapid reload head was designed to help with faster, easier reloading of the line when it's been completely used within the head.