Class 66 Metal bobbins are the correct ones to use in a 201. The plastic ones "should" work if they are a quality brand but they vary a lot.
The problem is most bobbins made today come from China and their quality control is hit or miss. Some of the bobbins work good, others are terrible.
Sew-Classic has the nicest Class 66 metal bobbins I've seen. I just received an order for 50 of them. All of them are as perfect as you can get.
Joe