Eikon Conventional Blue Back Timing Spring. 

Individual timing back springs are slotted for easy installation and removal. Designed to work with any conventional armature bar. 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: 44.5 mm (1.75")