I thought it was a bit of both, Spey cock for lighter flies and heron for darker ones.
Spey cocks are now regarded as extinct and it was the feathers from the side of the tails that was used.
I love Spey style flies and your hackle isn't really long enough Ben, you will struggle to get schlappen to suit
, I've tried
I have some heron
and like using pheasant body feathers. Some of the larger back feathers on a coot
Are a good Spey hackle too
Interestingly H's suggestion of GP body feathers is a good one and formed the basis of the very similar Dee style flies
Also tied with long flowing hackle and also dating back many years.
I guess any feathers which are long fibred and soft enough will be usuable
Webby feathers are nice for this like flank from teal and mallard,
So I think all the suggestions so far are valid
It will be interesting to see what you come up with next
Sandy