That particular "gramota" (birchbark letter) is the one most often reproduced in Russian history textbooks. Another one that is mentioned marginally less often looks like this:
http://gramoty.ru/index.php?no=46&act=full&key=bb
...and goes, approximately, "an ignorant one wrote this, a thoughtless one said this, and you who reads this is [presumably expletive fragment torn off]". Or, in modern parlance, "IF U READ THIS UR A FAG".