Is a watermark the only way to tell the difference between these two special delivery stamps?
Here is a direct answer:
2. On cover or piece use where the dated postmark (of E-4) predates the issuance of E-5.
3. The so called "DOTS" of E-5 Plate Number 882 (E5a).
4. Line through "Postal Delivery" variety of E-5.
5. Any stamp with selvage including a plate number other than 77.
#3 and #4 do not occur on E-4.
#5, Plate 77 was the only plate used for E-4, but was used for E-5 as well. Any other plate number used on E-5 is not found on E-4.