Most visible difference is the flange of the muffler where the two bolts 8 go into the case. The newer muffler has an arched flange edge, the older one on the bottom a flat flange edge. I believe the older mufflers weren't as deep as the new ones, requiring flange 11 to seal againtst the cylinder. Newer mufflers including the aftermarket ones will fit against the cylinder on all saws. For the early 064 the muffler might stick out a little on the front, requiring the thickness of the flange 11 behind the bolts 8