The original 3520 (some times called the Osolnic lathe after Rudy) was made in Tennessee. The A was then taken over seas. The new C looks nice, and the wedges for the tool rest post is a nice addition. They also mill a flat on the back of them and use set screws to keep them from rotating when you take one tool rest out and then put another one in. Some thing I have been trying to figure out how to do on my Robust. Other than that, not much more than the B model.
I am not a fan of the 'extended' housings on the headstocks. It may be nice for getting to work on the bottoms of twice turned bowls, but by extending it, you get vibration issues because it is cantilevered farther away from the base/headstock.
I did prefer the A model for speed ranges, 0 to 1500 on the low end, and 1 to 3000 on the high end though the 3200 on the B model wouldn't hurt. Mostly for bowl turning, on the low end, which is better for coring and heavy roughing, 1200 is too slow for a good part of the finish cuts I make, and on the high range, it is not as good for coring.
Still a good lathe. There is not a huge amount of competition in the lathes from 16 to 24 or 25 inch range. Best bets to me are Oneway: long bed but you can turn out board, Robust with sliding headstock, and Vicmark with the pivoting headstock. I don't like long bed lathes for bowl turning because I don't want to bend over or extend my arms way out away from my body, especially when turning out the inside of a bowl. I am looking seriously at the Vicmark...
robo hippy