Remington 870 with the ISS (J-lock safety)
Special thanks to RomeoPapa for posting pic of the ISS (J-lock safety) of Remington 870 on Remington 870 Forum:
Here ya go. As shown, with the pointer aligned with the white dot, the safety cannot be moved to the “fire” position. You have to insert the key (the green thing) into the “J” shaped opening and rotate the pointer to the red dot. You can then take the safety off.