There was and maybe still is, a discussion on this on the BBS site. The solid sided generator seems to go back farther than originally thought. Mine has a 9-56 date on it and is probably not the original, but might be period correct. It was even stated that it could have been a factory warranty replacement in the cars first years of life. It's staying on what ever the case.
How did I pull the brushes back? Before putting the end plate on, I first pulled the springs back and laid them against the side of the brushes, which held the brushes in the open position, then when 3/4 back over the commutator, I released them with a dental pick. After that, everything just slid back together. PJ