A flat mount disc caliper on a post mount frame? There's an adapter for that - CyclingTips
Flat Mount Disc Brake Adaptors - Velo Orange
Tektro Flat Mount F6 160mm Rear Adapter | Alltricks.com
Found: A.S.Solutions Flat Mount Adapters for Post Mount and IS frames and forks - Bikerumor
Post to Flat Mount Brake Adapter – Wolf Tooth Components
SRAM Flat Mount Disc Brake Adaptor | Jenson USA
FF-5 Front Flat Mount 140/160mm Adapter – TRP
TRP FF2 Disc Brake Adapter - Front Flat Mount Fork to Post Mount Calip – Ride Bicycles
Shimano Flat Mount Disc Brake Adapter - Components
Add Flat Mount Brakes to Post Mount Frames and Forks with Our New Post – Wolf Tooth Components
Amazon.com : SHIMANO Flat Mount Disc Brake Adapter Front, Flat Caliper to Flat Mount, 140/160mm : Sports & Outdoors
Shimano Flat Mount Disc Brake Adapter - Bike
Amazon.com: SwishTi Cyclocross Gravel E-Bike Bicycle Bike Flat-Mount Disc Brake Adapter for Flat-Mount Forks and Post-Mount Calipers use w/2pcs FM Bolts : Sports & Outdoors