Eikon Tru-Spring Blue Back Spring 

Individual back timing springs are slotted for easy installation and removal. Designed to work with any Tru-Spring armature bar (not with Conventional armature bars). Made of blue tempered high carbon spring steel.
Available in thicknesses: 0.016", 0.018", 0.020", 0.022".
Springs are flat and require bending.
Width: 13 mm (0.5")
Length: 30 mm (1.2")