Both O'Connell,
Production Log p. 211 and Foreman & Harvey,
Combat Diary p. 232 ascribe Fahrmann's loss to the Thunderbolt of 1st Lieutenant John J. Usiatynski of the US 358th Group.
The following source, however, gives Usiatynski's victim on 8 April as Uffz. Wolfgang Severin:
www.armyairforces.com/rss-m125308.ashx
And this source gives Wolfgang Severin as a member of JV44:
www.me-262.de/home/jagdverband44jv44flu.php
regards,
Jim Geens