See:
https://stampsmarter.org/features/Id_US_SD.htmlBecause of the curved lettering of "Post Office" you can narrow it down to E2-E5.
Because of the horizontal line you can see below the shadow under "TEN CENTS", this narrows it down to E4 or E5.
Next you need to determine if the stamp has a watermark. Hold the stamp up to a light or place it face down on a black card and see if it has a watermark. Most likely it is an E5.