Just as an aside, I have scrapped a few of these converters. The input and output use identical sockets, so they are actually illegal to connect to the mains here, and probably elsewhere as well, as the input is via a mains plug and not a socket with the strong possibility of touching a live plug.
The transformer in all I have dismantled has a 3 part winding, with it being 55VAC, 110VAC and 55VAC respectively. Thus the case if 220VAC is applied to the unit is the output has one pin at 55VAC and the other is at 165VAC, with 110VAC between them. This voltage is with respect to neutral ( which is basically the same as the protective earth potential wise, a slight variance due to loading and the finite impedance of the wiring in the house being a few volts on most cases, basically the same). This happens irrespective of how the input is wired, the pins just swap, but either way they are not SELV levels.
The other way, going from 110VAC to 220VAC, the 220VAC output is either 165VAC or 55VAC above the ground terminal.
Thus the best is to make a dedicated 3 core lead to bring power in, and have a 3 core lead to a socket to take power out. Connect the green/yellow leads together inside the converter, and add a short wire to a lug under one of the transformer bolts to ground the frame. Your output will still be non isolated, and add a notice that the output is non isolated, and should be treated with the same respect as 220V wiring and appliances.