I'm building up a road frame with IS disc mounts and would like to use the Ultegra R8020 groupset (with newest gen hydraulic discs). These disc calipers are flat mount, but in the product specifications, the Shimano website has the cryptic added comment "(available for IS mount with mount adapter)". I don't see any available flat-to-IS mount adapter from Shimano or anyone else. Does that comment mean there are substitute calipers that are IS-compatible or is there a flat-to IS adapter that I haven't found?