Texas John, I'll do my best to explain the situation regarding ESAs as I see it. There are people in this club who are competent engineers so they can then take apart what I write. The long standing fault with the original ESAs was their tendency to come undone and break the springs. A study of early MPHS will show some of the ways in which people tried to overcome the problems The series D ESA was a significant improvement and, if properly assembled, should not come undone. However, it has become clear with time that the force exerted by the original springs system is inadequate and that under any significant force the ESA goes from hard over one way to hard over the other. The Australian version with more springs was educational in that it did not move enough. This gives us a good reference system to work from, I.e. too weak to too strong. If we can measure the spring rate of each spring pair in the original systems then we can easily work out what strength springs are required to give us the required force.
You mention progressive springs. The mechanical design that Vic advocates would impart a progressive force from linear springs. I am not convinced that either solution is required, just some stronger springs.
Regarding the "redesign" of the ESA a few yeas ago; Ian can put us right on this but my understanding was that it was not so much a redesign but a correction to the drawings as there had been some design creep over the years and cam profiles had varied from the original.
When it comes to the use of "O ring string" to act as a resistive medium then I am suspicious. What shore hardness and what material would one advocate and who would ensure that the same material was used on each occasion? The way that I see it is that if we came up with the correct strength linear spring then they would be a trivial cost and it could be ensured that each batch met the specification. What would be the advantage of using o ring material of, possibly variable quality, rather than sticking to the original design, with D improvements, and just using springs.
I am away from home at the moment and cannot measure the spring rate of the original spring pair combination. When I can do that I can the find out from spring manufacturers whether a single standard spring could do the job or whether there is a problem meeting the spec with a single spring or whether something more complicated is needed.
This might all have been done before by Stu and I am sure he will let us know what happened.
The use of a single spring might work and I seem to remember back in the days of George Brown that he, or one of his competitors, used a square section spring but I have no way of knowing whether this was a term solution or not.