Steve
The trigger mechanism on that Flat Top and Full Barrel is virtually the same. The spring is bent down allowing the kill loops to hang down in position. There should also be a piece of cord on the end of the spring which passes through a hole in the centre of the trap.
The trigger is then inserted into the same hole, from underneath, with the bulk of the trigger inside the trap. The tightness of the trigger holds the cord, which holds the spring.
Moley, passing through the trap, knocks the trigger peg, dislodges it slightly, which releases the cord and so the spring, pulling u