All cheaper steel frames come with flat mounts. Genesis, brother, surly, etc. I guess it's cheaper to weld an IS mount to two tubes than to manufacture one with flat mounts.
I feel like I read something from either the fairlight or mason designer about how flat mount is a huge PITA compared to IS mount when you're dealing with the manufacturing of steel frames. because of tolerances etc
all the time, in my experience.
I really don't understand Bombtrack bringing out this frame with post mount. GRX won't even fit.