The "Steelfin" Fox .15 came out in 1958. The next generation was the .15X which has a drop-in liner. This was released in 1960 if I remember correctly. I think the XX was like the X except it had a better bronze for the shaftbearing. There was also a special XXX.
The X went through several iterations. The one in the lower left is the first X, note that it has no muffler provisions. The others shown mount the muffler with over/under screws. I also have one on an Akromaster that has muffler ears on the sides of the stack.
Note that the steelfins have no provisions for muffler. The one with the "square" fins is the first version.
George