== When?  What? == 

 "Various sources state that production started in 1964 or 1966. Explosia's brief historical document states it was 1964,[5] but most other credible sources state it was in 1966. Most of these also state that development was started at the same time, in response to a request from Vietnam for a counterpart to the US's introduction of C-4." 

 What does this mean?  Surely Semtex was the counterpart to C4, so how could they develop it at the same time as they produced it?