This is the ME-8104 Rotary Plastic Roller Arm Enclosed Limit Switch which is very useful for electromechanical machines. 

ME-8104 Rotary Plastic Roller Arm Enclosed Limit Switch:

1.  Product Name: Limit Switch
2.  Model: ME-8104
3.  Type: Rotary Lever Arm with Roller
4.  Power: AC 250V, 5A DC 115V, 0.4A
5.  Fixing Thread Diameter: 4mm/0.15"
6.  Roller Dimension: ~1.8 x 0.7cm / 0.7 x 0.3"(D*H)
7.  Lever Arm Length: 4.6cm / 1.8"
8.  Overall Size: 12.3 x 2.8 x 3.9cm/4.8" x 1.1" x 1.5"(L*W*H)
9.  Color: Black & Blue
10.  Material: Plastic, Metal & Rubber
11.  Weight: 82g