Hello,
Those pictures of the purple FM336 are actually mine haha. I was the second customer of this frame and I told Jack about this. They had 50mm tire clearance announced in their website which was false. Maximum is 47mm. Loca bikes (a bike brand from Poland that bought 500 frames) announces in their website 47mm tire clearance, that's how I noticed something was wrong with my rear tire clearance. 50mm at the front is okay. Definitely doesn't fit a ThunderBurt 2.1.
Best regards,