In Britten's Simple Symphony is the term "marcatiss." What is the definition?

I personally have encountered the same marking in the score for Peter and the Wolf, and while I can't find a direct answer, I have some theories. My first one is that it is simply another way to write marcato; that is an accentuated note similar to staccato but held for the entire length. My second theory may be going off on a limb, but it might be a combination of the words marcato and assai, in which it would just mean very accentuated.