What tap handle are you trying to use? Is the metric handle metal or plastic or wood? If it's plastic or metal you can probably just run a 3/8x16 tap into it to cut new threads. If it's metric it's probably 10mm which is slightly smaller than 3/8 so the tap should work fine to cut new threads
I am not sure whether metal or plastic threads, since they have not arrive yet. I only know they are from Europe, so they will be metric.
Thanks so much for the information. It is very helpful.