How to install flat mount road disc brake calipers on IS mount bike? I have custom MTB frame and I bought a pair of Shimano Hydraulic road shifters.
They have on Shimano BR-RS505 Road Hydraulic Brake Calipers Flat Mount when instead my frame takes standard IS 51 mount (+adapter of course for it).

What kind of adapter should I use?
I have noticed that this kind of calipers use a front adapter mount and it might work on the back too...

Thanks for your help!
As of now there is no commercial product you can buy to make this possible. An adapter that did this would be awkward to design because the flat mount caliper, in its native spatial position relative to the rotor and stay/blade, is trying to occupy some of the same space as the IS tabs. I believe it's physically possible but it would have to resort to being bulky and positioning the caliper quite far away along the rotor from where the tabs sit, which is a very negative design trade-off. It may even raise questions about how much leverage the tabs or mounting bolts can handle.