In the early days all oboes had ring systems for keywork. Later everything changed towards plateau systems. I have a question ons this : What was the reason for doing this ? And why this kind of evolution didn't took place on clarinets, they still have open ring keywork.
Another question : what are mainly the differences between soundquality if you use bladder, leather, cork etc... as pad material on an oboe ?
The system changed around the turn of the 20th century and is largely untouched from that Gillet System of 1906.
Many players were against it as it added so much extra weight to the wooden tube which was thought to deaden the ring and tone of the oboe. Goossens himself didn't like the idea at all but the system prevailed and now most players use it throughout the world. Some players used a half and half system with rings on the top joint and plateau on the bottom joint.
The reasons why it came into being were mainly to facilitate accurately tuned trills. For example trilling on a ring system between A flat and B flat or D sharp and E without the plates is not very pleasant. The tuning isn't great despite the speed of a trill, it is still very noticeable. In the UK where the thumb plate rules it was even worse in some ways. We now play mainly hybrid or dual versions that gives the best of both worlds - we can have the conservatoire B flat and the thumb plate middle C. There is a better tonal match for these notes.
Another benefit is thought to be ease of playing - and that finger movement need not be as precise - but I don't really subscribe to that as it is all too easy to slide off the little hole in the plate. This is especially when the system is not well maintained and light to finger pressure. Undue force is often used to hold a key down to overcome poor adjustment and the finger slides off the little hole and the note fails.
Sound is certainly governed to an extent by the pad material. Some players like to have skin pads on the small keys - say between first and second fingers left hand as an example. Those little pads are said to benefit. From a sealing point of view I do find cork to work well if it is well padded and adjusted of course. Maybe skin is more forgiving to poor workmanship? With thicker bored oboes the pipe is just a little longer and I don't hear much difference between cork and skin.
Each maker tends to stick to what works for them them
I have a few oboes of different makes. My Rigoutat uses corks except for the bottom notes these are skin (bladder) and curiously a skin pad on the F key! Although the newer instruments have leather pads on the bottom.
Buffet Greenline uses cork and leather on the bottom. Loree uses corks throughout and Marigaux corks and leather on the bottom. The sound of these oboes all sound like me playing them!
The sound quality is influenced more by how close the pad is to the tone hole, how wide the tone hole is in diameter and how much undercutting of the tone hole there is. It is a combination of the artist maker to get these things in balance. Most players go for good seal first and marginal tonal differences next I think. Tonal difference is hard to judge though so I can't really describe it to you. It is certainly not enough to say that cork is bright and skin is darker!
Hope this helps. An interesting question.