The T01 trigger used 3 spring-loaded ball bearings to lock the piston rod into cocked position, while the T05 uses 2 sliding plates with oval holes in them that the piston rod fits through. The piston rod ends are different, which is why you need to change the piston as well as the trigger group when going from T05 to T01 and vice-versa. IIRC, the T01 has 2 screws in front of the trigger blade, while the T05 doesn't. Some Dianas are also stamped with the trigger type.