I've researched every Stampley who served during the War to the best of my ability, primarily in the Archives in Jackson, MS and with some help from William Hadskey, now dec'd. Jefferson L's CSR (Compiled Service Record) shows that he died on the 2nd of July. I have found the LDS info to not always be correct, and I have seen the date you mentioned as well.
My ggg, Thaddeus Samuel Stampley, enlisted in 1862, just a few months short of his 15th birthday, in the 4th Mississippi Cavalry (aka, Powers' Regiment), Co. G I have a copy of his widow's pension application of 1927. He died in 1902.
'Thad' was a son of Samuel Phadia and Sophia Compton Stampley.