When taking a good look at this model you see that it is a modified TX 125 model. When you put both models next to each other you can clearly see the similarities. (Pictures: above the SX 125, below the TX 125).
The buddyseat is new and the techical details are similar to the TX 125. The SX was manufactured for one year only in a total of 6,500 models and was followed in 1975 by the SXT 125 model.

Source: Aermacchi-De Gevleugelde and AMF-Harley-Davidson 1972-1978 by Jaap de Jong