To be honest I'm not sure what Syn's main strap is. I think you're referring to the one with a skull that's positioned just under his shoulder right? That's what I seem to find on google images when looking for Syn's strap. I do know that one of the school's students ( @Dominik Gräber ) has made a strap for Syn. White and gold to match his "White Goddess" signature that turned out incredible! Looking over the images on google I can't seem to find any info on it. It could be a custom one made for him, it could be an older strap that is no longer in production, or he could've just bought the skull pin and put it on a strap himself. Either are possible. I hope you're able to find out if you can get your hands on one!