A "simmaniya" (in Hebrew) is what we used to call the dedicated cards inserted into books to mark the place you stopped reading.
They could be quite elaborate hand made or hand decorated objects - and were of course - used again an again.
In other words: a simmanya card is a bookmark. And the "mo" would be the Ashkenazi pronunciation of the "ma" sound.
I don't what's special about the bookmark mentioned in the letter - perhaps the writer was an artist perhaps it's a sentimental issue, maybe a totally different meaning of the term with which I am not familiar, or even a code the reader would understand and the censor would not fine suspicious.
But it could be a beloved bookmark card.