Master Lock 387 King Pin Trailer Lock
Solid, cast aluminum alloy withstands physical attacks
Bright red finish and red flag are visible to the driver from a distance.  Easy carry handles 
Note: Available keyed alike and master keyed for your entire fleet
Lock, protected by the housing, resists common physical attacks:  Shackle cannot be cut, Hardened steel bottom plate resists drill attacks.  Rivets are countersunk making chisel attempts fruitless