Generally speaking, older Edwin M Knowles plates are more valuable than newer ones. Plates that are in good condition with no chips, cracks, or repairs are also more valuable. Rare plates that were produced in limited quantities are also more valuable. Plates that feature popular subject matter, such as historical figures, animals, or landscapes, are also typically more valuable.
If you have an Edwin M Knowles collector plate that you are interested in selling, it is best to consult with an appraiser to get a sense of its value.