STM32

https://en.wikipedia.org/wiki/STM32#Part_number_decoding

STM32F103RCT6

https://stm32-base.org/boards/STM32F103RET6-Generic-Board

The original Espruino's firmware (see https://www.espruino.com/Original) can be flashed via the USART breakout near the USB connector.

See also https://github.com/espruino/Espruino/blob/master/boards/ESPRUINOBOARD.py for peripheral connections in case you'd want to replicate them.

devices = {
  'OSC' : { 'pin_in' :  'D0',
            'pin_out' : 'D1' },
  'OSC_RTC' : { 'pin_in' :  'C14',
                'pin_out' : 'C15' },
  'LED1' : { 'pin' : 'A13' },
  'LED2' : { 'pin' : 'A14' },
  'LED3' : { 'pin' : 'A15' },
  'BTN1' : { 'pin' : 'B12' },
  'USB' : { 'pin_disc' :  'C13',
            'pin_dm' : 'A11',
            'pin_dp' : 'A12' },
  'SD' :  { 'pin_cs' :  'D2',
            'pin_di' :  'B15',
            'pin_do' :  'B14',
            'pin_clk' :  'B13' },
  'BLUETOOTH' : { 'pin_tx' : 'A9',
                  'pin_rx' : 'A10' },
};