Donald, I was always led the believe that the original fly was tied from the underparts of the Tup or Ram which had been dyed that yellow/pinkish tinge by the urine of the Ram. I think Oliver Edwards mentions this in one of his books or videos.

How they got it I have no idea. Chasing a ram around a field to snip a lock of hair from around its nether regions would not be my idea of fun!! Nor its I have no doubts!!

Veniards sell tups seal fur but that is an orange/pink.