I see no reason to doubt the authenticity of your coin. The obverse of your coin appears to match the second coin from the bottom on this page
http://beastcoins.com/Sasanian/Sasanian
The description for that coin is
Arab-Sasanian, Anonymous Issue, Abbassid Governors of Tabaristan, AR Hemidrachm, 136 PYE (Post Yazdgard Era = AH 170 = 786/787 AD), Tabaristan Mint