You think the envelope proves it's valid? You're kidding, right?
And whose handwriting experts? The government's? Would they be like the government's anthropologists? And the government's scientists who tried to rationalize the collapse of the Towers, including Building 7 which was never hit by a plane, by concluding that it was felled by "office fires"?
And why would Oswald write his own name in Russian on the envelope? And even if he did:
I translated Oswald to Russian using Google Translate:
Here is what's written on that envelope:
But, read what I wrote again because I gave cogent reasons why the authenticity of the letter should be rejected, and you haven't responded to any of them.