I'm not discounting your experience with Thai Stick. Mine is somewhat similar. I think we are roughly the same age, from the same country. I had sticks that were supposedly dipped in opium. I didn't find them any different. I did not smoke real opium (mixed with pot) until a few years later, in '83, and I remember thinking at the time thinking that it was not a high I really liked as it was too relaxing. I liked upbeat highs. Btw, I mixed the opium with cannabis myself.I never claimed I was an expert on Thai sticks Chi13, but I have grown Thais for over 4 decades I also sought out the imported Thai lots each year until it ran out regardless of how much weed I had. Plus, I had contacts and friends and I had a good idea of what was being imported in or was around.
You few clearly did not experience these Opiated Thai sticks and that's okay, but some of us did and like I said they were not that common.
Just because you or a few others didn't come across them don't automatically mean they never existed.
As I've said bottom line for me is no evidence from those involved in Thailand, in fact no one involved said this happened. There is also no evidence from drug seizures at the time and the press would have been all over it. They love their scary drug stories.
There were a lot of myths about cannabis back then. The opium dipped Thai Sticks was one. I know from experience you won't change you mind on this, so believe what you like. Unless you had the product analysed then it is guesswork.