Have you tried checking the Ellis Island records for your great-grandfather's arrival? I checked for Trelc, Treltz and Trelec, but there are no entries for the first two versions, and none that fits for the last one. According to the Slovenian telephone directory, there are over 15+ Trelc in the directory today. Unfortunately, most appear to be scattered in different towns, though the majority seem to be in the area north of Ljubljana, around Kranj, Radovljica etc. There are quite a number of parishes in this area so you will have to narrow down the possible place of origin for your Anton Trelc. I suppose you have tried all the available records in the US to determine where Anton was born? If he was still alive in 1930, the census for that year might give his place of birth. It was one of the questions asked at that time. Hope this helps.