https://www.history-matters.com/archive/jfk/wc/wcvols/wh16/html/WH_Vol16_0029b.htm
Duh !.... Everybody knows that the letter was addressed to the Russian Embassy. And it's common knowledge that the FBI was reading all of the mail addressed to the Russian Embassy. So it wasn't just the Russians who received information......
When Lee was in Minsk he often sent letters ( sometimes picture post cards) to his mother, that contained photos of places in Russia. Those photos revealed places that were of interest to US intel. Lee knew that his letters would be read by the Russians before they left the USSR and by US intel when they arrived in the US.
So Lee knew how to send seemingly innocuous information in a letter to his mother.
Now then,... let's display a bit of alert intelligence shall we Charlie?
Did Lee inform the reader that " Nichilayevea " was living in Dallas??