The Iron Bell isn't based on any particular Muff circuit. Instead, Mojo Hand tweaked it to be a platform that is wide, yet focused. Unlike the Colossus Fuzz which is based on the Sovtek/Civil War era Muff, the Iron Bell is voiced to be very open and rich, with an almost 3D like harmonic quality. Available at JetCityMusic.com.
http://www.jetcitymusic.com/index.php?page=shop.product_details&flypage=vmj_naru.tpl&product_id=726&category_id=35&manufacturer_id=66&option=com_virtuemart&Itemid=89