As DrZzs call this; Jumper Holder Thingy
Some of the new Sonoff Basics ( and probably others ) are being delivered with solder filling the serial programming interface.
So if you want to flash it, you need to either remove the solder or you will need some dexterity to hold your 4 wires (Vcc, Gnd, RX, TX) to the proper point, depress the button, plug in your USB UART TTL adapter (3.3 volts only), release the button and flash the device. Do-able, but difficult.
That's where this clip comes in. Print it, put in pogo-pins in , connect to your USB UART TTL adapter and it will hold the pins to the right connectors.
The pogo pins I am using are P75-E2 Convex Tip Metal Spring Test Probe Pin, 1.5 mm Diameter. Link here https://www.amazon.com/gp/product/B018YPQ6ZK/
Update: I found over a little time, the pressure was reducing on the pogo pins to the board, resulting in having to add a rubber band or squeeze the pins a little while programming. So the result is V5. I have added a 3/8" x 2" (10mm x 50mm ) Barrel spring other types would work, just dont want too much pressure. This seems to work very nicely. I will leave v4 in here also.
Note: Rotate model so it sits flat in your slicer for printing.