Final answer:
The Staff model of HMO was specifically included in the HMO Act of 1973.
Step-by-step explanation:
The model of HMO that was specifically included in the HMO Act of 1973 is the Staff model. In the Staff model, the HMO directly employs its own physicians, who provide care exclusively for HMO members. The HMO Act of 1973 aimed to regulate and promote the growth of HMOs, and the Staff model was recognized as one of the primary models of HMO at that time.