There are some bobbins out there that sorta look like Class 66 bobbins but are for another brand of machine. I don't know what they are. Then there are some Asian made bobbins that are just plain junk. Their sizes are inconsistent and they sometimes wobble to the point you can't wind them on the winders.
As long as it's a genuine Class 66 made to Singer specs it will work OK in your 201.
Joe