Transfer

uint index = [whichever device you want to check];
ETrackedPropertyError error = new ETrackedPropertyError();
StringBuilder sb = new StringBuilder();
OpenVR.System.GetStringTrackedDeviceProperty(index, ETrackedDeviceProperty.Prop_SerialNumber_String, sb, OpenVR.k_unMaxPropertyStringSize, ref error);
var probablyUniqueDeviceSerial = sb.ToString();