It is a great fly of orgination by one Cannon Greenwell. 1850s
Its origination to represent the Olive Duns which are a speices we do not see here in the US. They frequent the UK freestone streams and Chalkstreams.
There is some controvery so far as the correct shade of Pearsalls thread used for the body, some say yellow, ( and there were a numebr of different shades at that time ) waxed, others olive. Cobblers wax in those days was black.
Hackle shade also, which can be of choice between a light shade of furnace to very dark which would be more a Cock-Y Bonddu, either way it should be a hackle with a black base, color and black tips.
One of my all time favorite flies more so as a winged wet fly and then a soft hackle.
It produced for me many years ago when fishing my the local river the famous Welsh Usk one of the best day ever fishing for wild Browns on that particular river where fish are difficult to catch and not many fish per mile of water.
It is one of the best known UK flies and called there the Greenwells Glory.
Nice ties you have there also.
Davy