Multi-gang sonoff light switch, aka TX T3US1C, T3US2C, T3US3C. Also see the tasmota template.
On mine the button board pops off the relay board so you can flash it with a USB adapter, then plug it back into the mount on the relay board.
Seems to be some sort of protection in place to prevent reflashing with the espressif flash/download tool:
ESP8285 Chip efuse check error esp_check_mac_and_efuse
Both the Tasmotizer and the esp8266_flasher.exe worked for me. I've added the esp8266_flasher.exe to the downloads folder (below), the tasmotizer is available from github. The esp8266_flasher reports "Failed to leave Flash mode" when finished but works properly.
Using with MppButtons.bin for multiple switches+relays:
WifiLED GPIO13
Button1 GPIO0 (1,2,3 gang)
Button2 GPIO9 (2,3 gang)
Button3 GPIO10 (3 gang)
Relay1 GPIO12 (1,2,3 gang)
Relay2 GPIO5 (2,3 gang)
Relay3 GPIO4 (3 gang)
E.g.:
WifiLED 13
Buttons 0,9,10
Relays 12,5,4