I have always wondered why hooks had TU or TD eyes. My investigation led me to believe that both directions of bending were done to accommodate snelling of the then prevalent gut leaders. The TD eye accommodated the argument about the pull being more in line with the hook point while the TU eye accommodated those guys that wanted more useable gape. With the advent of nylon tippets in the 1950's the Turle knot was developed which also provided a straight line pull on the hook shank with either eye shape. I pursued this to some extent with the folks at Mustad and was encouraged by noting that we are seeing more and more new hooks with straight eyes and that the TD and TU eyes were mostly holding their popularity because of our inertia.
If anyone can add to my superficial investigation I'd like to hear from them.