And..... I don't think the Wika will fit into the hole. The FX gauges are pretty small diameter and longer. The stem nipple of the FX gauges are longer and seal against a plastic bushing/washer inside the receiver. The Wika's are shorter so if you decided to go that route you'd have to figure out the best way to get a good seal there. Assuming your fill gauge is closer to being the correct of two you could always order a couple of new FX gauges and go with the closest one. I'm reading both tank pressure and regulated pressure on mine and fortunately they are both within 5bar of my test gauge.
Jimmy