only mesh (STL,OBJ) is available for download. Can't really use mesh, since I would have to make pretty deep alterations to make it work with snapmaker. STEP file would be much better for this.

I don't see the AutoClippy model here - https://www.printables.com/@wjsteele_27897/models

@Bill could you help?

https://www.printables.com/model/1015615-3d-chameleon-adoptation-for-snapmaker-20-single-ex

More screenshots on how to setup the Arduino IDE to burn the bootloader first. Using USBTiny https://www.amazon.com/gp/product/B01CZVZ1XM/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Flashed SelectorFirmwareMk4.ino.hex to MK3 with USBTiny (last programmer used in the video) - flash ok, but board doesn't work, it turns on, but if I press the switch - no reaction. Should I use different firmware?

What file did you use? I flashed SelectorFirmwareMk4.ino.hex and 3DChameleon doesn't work...

Did you upgrade mk3 to mk4? What firmware file did you use?

Just sent the email.