I just purchased a used Leader 735R frame on ebay but still need to get a fork and headset to match. According the the Leader website, this frame has an integrated headtube and specs "16/cc", which I've never heard of until now. I did some googling and found that quite a few other bikes spec 16/cc with various brand headsets (Shimano, FSA, etc), but when searching headsets alone, have yet to find and actual headset that mentions this type. Some of the other Leader frames spec Cane Creek IS, so I'm guessing there is a difference.
That kind of makes sense, although the website says "16", not "I6", and there are other bikes out there who spec the same "16/cc", but from FSA and TH industries (I'm sure I came across shimano to, earlier):