I copy/paste from another 3D, I hope you need it:
If you're looking for genuine parts, the correct crown and tube for the 1680 is the current crown and tube assembly for the 16610. Part # 24-703 and 24-7030. It is a 7mm crown.
Older crown and tubes like the 24-700 and 24-702 could also be used, but they are more expensive and the 24-703 assembly is the latest and best unless you're trying to return an old gen back to it's original condition. Rolex updates older subs with the 24-703 assembly when servicing older watches.
When buying on ebay, you have to be careful, because other numbers are often times used and older crowns don't fit the newer tube. Used 24-703 crowns are easy to find and then you can get an aftermarket tube from a place like Clark's Watch Supply (Nostalgia2000) on ebay and it should work out OK.