What I want to say is that Ltwoo does not adhere to any known standards, whether it's flat mount or post mount. Ltwoo has designed its own specification and provides a bracket with the kit to install it on most flat mount frames. Therefore, you will find that the included screws are M5 size, not M6. Additionally, this Ltwoo-specific specification was not initially designed to accommodate post mount frames, meaning it cannot be installed on any post mount frame.
As for the second question, it is actually easily solvable. Any Shimano hydraulic disc brake caliper compatible with mineral oil systems can be used with Ltwoo's control lever, including but not limited to models like MT200, M6100, R7170, GRX400, and so on.
You just need to ensure that the syringe, olive head, and hose are compatible. I recommend using BH90 components across the board because BH90 hoses can be directly installed on all BH59 compatible components. As long as you ensure proper hose installation, you don't have to worry about any compatibility issues.
I have paired my GR9 control lever with the UR300 flat mount caliper using BH90 hose, syringe, and olive head, and the braking performance and feel are excellent. I have been using it smoothly for nearly 5,000km without any issues of brake fluid leakage or loss of braking power. So, feel free to boldly pair them together.
Thank you so much. I came to that conclusion about the L-TWOO mount being oddball and
none of the plates compatible with post mount. A shame the seller specifically told me that it was post mount only. I'm hoping that was a language issue not bad info. (I will be telling them)
Also great info on confirming compatibility. I found a pair of post-mount RS785 calipers that will mount without an adapter nearby for cheap so that should work. The BH90 recommendation is also quite helpful. The Shimano spec sheets seem to indicate both BH59 in some configs and BH90 in others and who knows what's best with the LTWOO levers.
So BH90 it is. I've heard the responsiveness is generally a bit better with 90s regardless of the design spec in Shimanoland anyway. Just have to wait for hose since none of my local shops have it in stock. (Who'd have thought the calipers would be easier than the hose?)
Thank you to the others on this thread for also and convincingly confirming the applicability of Shimano calipers. I'll be back, hopefully with a success story and tips if all goes well.