The Proton C is an ARM STM32F303xC based drop-in replacement for the Pro Micro. It uses the same chip as the Planck rev6 boards, and Skull’s new ARM Clueboards.
You can find the Proton C for purchase at:
Support for Split Keyboards, RGB Underglow, and Backlighting are being worked on and should be available soon.
Some of PCBs compatible with Pro Micro have VCC (3.3V) and RAW (5VDC) pins connected (shorted) on the pcb. Using the Proton C will short 5V power from USB and regulated 3.3V which is connected directly to MCU. Shorting those pins may damage the MCU on the Proton C.
So far, it appears that this is only an issue on the Gherkin PCBs, but other PCBs may be affected in this way.
In this case, you may want to not hook up the RAW (5VDC) pin, at all.