You can't buy new ones MIJ (Made in Japan) anymore. Correct me, if there is still some secret place though.
You were not able to find answer, because you have not tried hard enough, or there just isn't any
In a nutshell, some say there is a difference(minority), some say there is not. The most people agree there is no difference in terms of the produced image. The coating has got different colours, that's for sure.
As for the visible difference? I don't think there is a way you could tell them apart looking at the sample photos.
You can try this nice thread:
https://www.pentaxforums.com/forums/122-lens-clubs/59538-fa-limited-club.html
Some people write down if they've got MIJ or Vietnam copy. You can also ask the authors if some image intrigue you in a special way
There are some amazing photos there, enjoy the ride~!
Let me also leave this here:
It is a part of the conversation I had with a Japanese seller, when I was hunting for my 31mm
Quote: It's a good move to buy a new lens.
For the record; In Japan, it is said that Pentax lenses made in Vietnam are better than the lenses made in Japan. It's been 8 years since Pentax stopped making lenses and cameras in Japan. If you buy a Japan-made lens, that's 8 years old at least, 8 years old technology. Up until 8 years ago, Japan's Pentax assembly line were full of aged or high school grads who did not find the better job anywhere. In Vietnam, only young and decent people work for the Pentax factory. Assembly line is strictly controlled under the supervisors sent from Japan who say workers in Vietnam are eager to learn and work harder (than the Japanese workers.) So do not trap yourself in a "made-in-Japan" myth!
This may help as well:
http://ricehigh.blogspot.com/search?q=vietnam
I still got MIJ Silver 31mm though, and I plan to get 77mm and 43mm as well. I am superstitious, weird and I have my own ways of dealing with life
I do not recommend nor advice following my steps
Trust your instinct, do what you feel you want to do. As for the difference in images, not worth worrying, you will not see any (unless your mind will play you).