Since no one has offered up any answers to your question, I'll make an attempt.
In the art of hook making there was a time that makers could not produce an eye on a hook. Flies in that time period were tied with a gut material loop tied in, aka: traditional gut eye salmon flies.
As hook making developed a loop (wire bent back on itself) was made in the wire to form and attachment location. The wire loop was seen as a problem because the leader was able to shift position when presenting the fly in heavy current.
The design was altered, to bend the loop up and out of the way, so the leader could be tied in through the loop and secured to the head of the fly. A turle knot was used with the loop as a stop for attaching the fly to the leader. This allowed the fly to swim in control with a straight pull and connection to the fly.
Fly control is key in salmon fishing, as in all fly fishing....