6-Pack Danly Die Springs. 

SPRG.0.500X3.00-RED-HVY-DUTY

Wire Type: Rectangular

A - Hole Diameter: 1/2(in) 12.5(mm)
C - Free Length: 3.00(in) 76(mm)

Spring Constant: 7.70(lbs per 1/10in) 13.6(N/mm)