The Zinc alloy ZAMAK #5 has a higher copper content than the ZAMAK #3 which improves it mechanical properties.   

• ZAMAK #5 has 4% Aluminum, 0.055% Magnesium, and 1.0% copper.

The alloy meets all the mechanical, physical, and fabrication requirements for the elbow housing.

But the  ZAMAK #5 alloy cost is marginally higher than the ZAMAK #3 with no appreciable performance and manufacturing benefit.