While it's true that flashbacks can result from any number of stimuli, even we veterans can't be sure what might result in our next flashback, much less anyone else's. My wife, who teaches fitness to older adults, was told when she was training that she should never play "sad songs" that might remind her students of loved ones who have passed.
My wife was quite troubled by that advice — she is a very empathetic person who would never knowingly cause emotional pain for anyone. But she decided that she cannot possibly know what song might be considered sad by her students, and decided to continue playing songs that best fit the exercises she is using. I get that.
I once went straight to fight-or-flight when I saw a guy carrying an M16 across the street in front of my apartment. That's a bit odd, because I never saw an M16 in Vietnam. I was a hospital corpsman serving with the Marines early in 1966, and the Marines were using M14s. In addition, I soon realized that the M16 I saw was a toy M16, about half the size of a real one.
My PTSD has also been triggered by a Vietnamese "peasant" wearing a conical straw hat in a style show that one of my Vietnamese students had invited me to, and by a scene in the TV show, Magnum P.I. But it would be pointless, and fruitless, for me to try to avoid seeing military rifles, black pyjamas and straw hats, and TV programs in case they trigger me. Fortunately, I've had enough psychotherapy to at least understand why I am so easily triggered, and what steps I can take to avoid the worst outcomes.
At the same time, if you are planning to give a military model to a veteran, it behooves you to try to determine from the veteran himself or herself, or from a family member, whether the gift might result in an unplanned outcome. Family members usually have a good sense of how trauma has affected their loved ones, and what gifts might be inappropriate. A gift I certainly would not want would be a diorama showing Marines in the middle of a firefight. Been there, done that, don't ever want to do it again!
It's also worth noting that the human mind is capable of great plasticity. I am currently building an Italeri UH-34 D Seahorse helicopter like the ones that ferried me into battle, out of battle, and to a hospital ship. No flashbacks, at least so far. Maybe if I make it sufficiently detailed…
Bob
P.S. Please do not thank me for my service. I didn't actually volunteer to go to Vietnam. I joined the Navy not even aware that hospital corpsmen could be seconded to the Marines. And, like most Americans at that time, I had no idea that what seemed like a minor guerrilla war in Southeast Asia would blow up into the Vietnam War. I did, however, believe in our mission, at least for a few days. I soon realized that nothing positive could come out of the war; I was fortunate to be badly wounded after only 37 days, while I was trying to care for a badly wounded Marine.