It's a relatively common stamp, so it's worth little off cover (catalog $0.75); it's probably best to leave it on the cover (catalog $0.85), as even without a cancel it would probably be worth a bit more that way and you also have some history behind the personality named on that cover.
Also, you should not confuse the stamp by using the term "mint". The fact is the stamp was used on a cover but does not bear any postmark. Most would refer to the stamp as being used uncancelled.
A quick genealogical search revealed that:
Quote:
Isaac Heston Worstall resided near Lumberton, Solebury Twp., Bucks (Co.), PA and later resided in Center Hill. He was an officer in the Civil War. He was married twice. He had two children by his first wife, Mary Jane and six children with his second wife, Amy. The sixth child with his second wife was one "Emma E. Worstall" of which your cover is addressed.
Emma E. Worstall was born May 15, 1870, and married William Wilson on November 15, 1897, residing in Hemet, CA. They had three children. No reference was made as to her date of death.
It is noted that many of the Worstall family members were Quakers.
Since the stamp shown on cover was issued on October 1, 1883 and Emma Worstall married on November 15, 1897, we can pinpoint the approximate date of that cover within that 14 year period.