I've come across 'forsaid' hundreds and hundreds of times in wills etc.
It's just usually used to refer back to something that has been mentioned before - could be people, places, etc
As we can't see the whole page, or perhaps more than one page, to see what it refers to in this case, I'd hazard a guess it is the mortcloth dues for a number of months, or years. As you say, an 'aforementioned period of time'.