Renato,
According to the write up in "Token Bijutsu", it has been thought that there were 3 generations of Moritsugu, differentiated by changes in the mei. However, the author of the write up posits that there is no real evidence for more than one smith doing the work, and that all 7 mei might be by the same hand. Considering that Moritsugu worked mid-Kamakura and that nothing was ever written about him or his work at the time he worked, it isn't possible to know for sure how many smiths were responsible for the work.
Grey