For decades now, 201 bobbins have been the standard to suit a wide range of Singer domestic sewing machines. In particular, Singer models 201, 99 and 66.
'Toploading' or 'drop in' simply mean the bobbins the dropped in and threaded into the area under the needle plate.
Metal bobbins (also available in plastic).
Sold in packets - 10 bobbins per pack."