Keep in mind that as with all external signs this is not going to be a rule. Not every twin flame is going to find the initial on their thumb.
I don’t want anyone staring at their thumb for hours and coming to the conclusion that it was a false twin flame just because they can’t see the initial.
It’s not that easy to figure out who your twin flame is.
I will say that I have spotted this, but my twin does not have mine on his thumb. Perhaps it doesn’t manifest for everyone every lifetime or it manifests in different ways. They could have my initial on his butt or something.