"Apologies, I didn't mean to belittle your work, it really was a moment of DUH!!! Why didn't I think of that in the first place <insert forehead slap!!> "

Sorry for the sarcasm - well, no, not sorry for the sarcasm exactly, because that's pretty much second nature: but sorry that you took me even remotely seriously. Trust me, I didn't feel belittled. I want you - and anybody else out there - to have the ability to accomplish the authentic effect. That DUH! feeling is very much how I felt once I put the pieces together. It's pretty plain to see, once you get it - and that's one reason I want the information out there. Even given that I had the insight uninstructed, it's too simple and after-the-fact obvious a principle for me to feel proprietary about it - I'm only rediscovering the common knowledge of craftsmen long dead; and isn't that what this tribe is here for? On top of that, a gentleman from the School of the Renaissance Soldier set the question in my mind of what the back looked like... so that once the hardware was in my hands, it offered the answer.
But I do thank you for the kind words.

Ah, nevermind. I get it. Now that I am correctly seeing he hardware I see the methods.

"How are the straps secured behind the scene?"

I think you're asking - correct me if I'm wrong - how the sword is held in place.
Review my photos with the following in mind (and study the captions): slide the 'buckle' up your strap between the two 'front side' loops of the figure 8, then curl strap backwards around to the central post (where the tongue would be fixed on a modern two-sided buckle). Fasten it in place there (I used a rivet), creating a free-sliding loop... a loop that slides up behind, not in front like all the commercial ones we see.
You asked about the 'free end'... There is no 'free end'; that's half the elegance of it: the whole combination sort of swallows itself. (The Sword-loop Ouroboros?)
Having made each loop this way, simply slide the scabbarded sword through.
Your leather needs to be just heavy enough that the 'buckle' doesn't slide too freely, yet flexible enough that your loop snugs down around the scabbard. You may need to do some oiling and tightening to get the right fit. (I did.)
Unless your sheath is pretty tight-fitting, your scabbard will now stay in place as your sword is drawn. (The more flexible your leather, the firmer a friction-fit you get... but the floppier your whole hanger is. Finding the happy medium is something of a balancing act.) Friction and gravity do the rest. If you're not planning to use the rig for more than one sword, you could even glue the scabbard in place for greater security; but I haven't yet found that necessary.
'Disappearing' the loop end inside itself also keeps there from being any loose straps to wear on your garments - important, given that it loops to the inside.

I've seen something like this design executed with the inside end of the straps simply left loose, i.e. on John Tripp's hanger, but this results in a gradual slide downward for your whole blade as gravity takes its toll, as well as leaving a set of dangling straps that appear nowhere in the originals. The otherwise excellent design of John's is also marred (sorry, John) by an adjustment strap leading from the the front lobe of the hanger itself, instead of extending straight out from behind the sword, permitting a certain needless 'wobble' factor - and again, not giving quite the look of the originals.

Of course (he added coyly), if this still isn't making sense, you could always commission a rig from me...