the B suffix corresponds to a French manufacturing (Bonnières-sur-Seine factory) The 15B are really common in France both in beige and black with the paperclip decal.
They were sold already electrified: this one was my grandmother's and she got it as a Mother's Day present in 1957 already electrified but I have seen some exact same models being mounted on a treadle table or cabinet and even some with both the motor and the treadle. However, it seems that they were made to be run with a motor since they have a "filled" balance wheel (as opposed to a spoked one) and these have a different diameter, something to do with making it more efficient with the motor power...