... this rope mechanism ( we shouldn't call it a knot, I think )...
This is a veeery difficult issue, which we had discussed time and again in this Forum, without becoming able to reach at any consensus. I want only to point out that a ( practical ) "knot" is already a rope mechanism - and perhaps it is nothing more than that.
... that exposed part can be gripped and pulled for adjustment through one of the overhands at a time, which is easier than if it was gripped with the same total force by only a single knot where you would have to overcome the same friction in one go.
Interesting point. However, when the loop is loaded ( even with a fraction of the total, final working load ), the first / "higher" overhand knot does most of the job, so what you really gain is not that much.
The problem is more essential : as I said, the continuation of the returning eye leg, when it goes through the first, more tight nipping structure, follows a
U-shaped, rather than an
L-shaped path. This maximizes the friction and the capstan effect at the tip of the U-turn, and so makes the job of the second / "lower" overhand knot a lot easier, but it also makes the adjusting of the loop a two-stage, more difficult and less quick procedure, if it is even partially loaded.
I believe I had tied and tried
all the single-knot adjustable loops there can be, and I had not seen anything more secure,
and TIB, than the
Pretzel loop.
I think the original knot was good enough, no need to improve it.

The first wheel was good enough ( one would even say "
perfect enough"

), but we have been improving it for six thousand years now...
