It's called "Ullsocken", which is Swedish for "The Woolen Sock". It's a simple fly, easy enough to tie, but hard to get the proportions right and there as many "rules" as there are flyfishers. The fly hails from River Mörrum in Southern Sweden and dates back to the 1950ies, but even though it's a quite famous fly, its origins are lost in time.
I tie mine with a red butt, a tail (not on the origial), pheasant cock red breast feather for tail hackle (brown cock on the original), bodies are black wool yarn, and the last two hackles are stiff, brown cock (as pr. thee original).
A friend swears by a bright red cock hackle for the tail hackle (and actually landed a good salmon on one of those yesterday).
It's strange how such a deceptively simple fly can fish so well, but it'd a fact that it does and there are lots of late summer and autumn salmon caught on it in Denmark and Southern Sweden. I've always had it in my box, but I think it was 3-4 years ago that a well known Danish fly fisher, Kim Sørensen, told me that he was using it to great effect this time of year. Kim knows his flies and his salmon, so I started using it as well, and I must agree.
Stiff cock hackles are important, I think - somehow I think move water and vibrate more than they flow in the current, and... Well, what do I know what the salmon think - I know the fly works and it works well.
So - for simplicity and in case you want to tie one, here's the original pattern:
Hook: Salmon hook, size 8-2.
Butt: Red wool yarn (DO NOT use your own socks, Paul).
Hackles: Stiff, brown cock.
Bodies: Black wool yarn.
And that's it.
I fish it much the same way as fish every other fly for salmon, no difference there. In the rivers I fish here in Denmark, it's important to get down to the salmon, so that (usually) means a 30g type 3/4 or 5/6 sinking shooting head and a short, stout leader.
Have a great weekend - I'll head down back to the tent to empty it a get ready for work (yes, work, yes, I'm working this Saturday, so please feel sorry for me :-).