Snap spring. Models: FS 80 - 4137 FS 85 T FS 120 2-MIX FS 120 FS 200 FS 250 FS 130 FS 160 FS 180 FS 360 - 4116 FS 420 FS 500 FS 550 FS 550-L FS 220 FS 220 K FS 280 FS 280 K FS 450 FS 400-K FS 400 FS 450-K FS 450-L FS 480 FS 300 FS 350 FS 310 HS 85 HS 86 T HS 86 R HS 87 R HS 87 T MM 55