Thanks for that link. I should have known to look at Hans' site or ask him. I'm guessing that AlanB is right about the properties. As for the spaniels hair, I've used hair from my dog. He was was a yellow Lab mix or from my yellow/orange tiger cat. Actually, any similarly color fur or hair will do, just not historically.
In his book, Donald Overfield wrote that, Austin wrote in a letter to Skues that it(the dubbing), "was a mixture of wool taken from the scrotum of a tup(ram), which no doubt accounted for the yellowish tinge caused by the urine, to which was added a pinch of lemon hair from a spoaniel, and seasoned with a small amount of fur from the poll of a hare, plus a little red mohair to turn the witch's brew a slightly pinkish shade." And Overfied writes on that, "Despite the dubbing material having been common knowledge since 1934, it is truly amazing what horrors are passed off to the angling public as Tup's. ... For such bastardization there is no excuse".
Anyway I think we've covered most of the history, LOL.