Does the machine have a motor encased in a "squarish" cover on the post of the machine? If so, it's a 15-91.
If it's belt driven, it's most likely a 15-90. Look at the bobbin carriage. If the "finger" on it points up and to the left, it's a 15-90. If it points up and slightly to the right, it's a 15-30.