REL is probably the first, certainly it is older than Knize Ten.
My guess is that Knize Ten is actually modeled after REL, come to think of it. Both have similar mandarin top notes, but after that the scents seem to diverge.
REL has a very short life span on me, alas! On me it is mostly mandarin, and well, that's it, but I have only the bottom of someone else's bottle in a eurodropper.
Knize Ten is probably as close to a "signature" scent I can think of for me. Long lasting mandarin merging into spices on my skin, not so much leather as perhaps tobacco when I did it in a mystery scent challenge, courtesy of Aztec.
REL you'll have to pay dearly for, of course. I haven't spotted testers of it at Douglas or at Blue Mercury.
Knize Ten is available relatively cheap right now at parfum1 but you can also get it at Caswell Massey. Have never seen it on the street.