Instructions:
The Red Tag is a classic European pattern dating back to the mid 1800's and continues to be a favorite trout and grayling fly for many anglers today.  It is believed to had originated by a man named Flyn in Worcestershire and was initially called the Worcester Gem.  An author named F. M. Walbran made it popular in the late 1800s. 

Tail:  Red wool.

Body:  Peacock herl.

Hackle:  Rhode Island Red (actually reddish brown).

There are many variations of Red Tag, or at least flies named Red Tag.  The only thing in common for most is the red tag tail.  An Orange Tag, also known as Treacle Parkin, is the same except for the orange tag.  It is a favorite of Hampshire chalk-streams anglers.  There is a popular White Tag in France and there is a Green Tag and Crimson Tag popular with Yorkshire grayling anglers.

Roger Woolley, the coauthor of "Grayling Salmo Thymallus" (1954) and other publishings, tied a variation he named Badger Red Tag that included a bronzed peacock herl body, silver tinsel tip, and badger hackle.  

The fly pictured on the right has a ribbed over and palmered red wool body.  I have fished this fly with good success for Alaskan grayling.  It is quite similar to an Australian fly named Red Bug except for the tied back hackle and palmered body.  The "Red Bug" is an Australian version of the Red Tag but stretches the limit to even be called a Red Tag variant, as does this unnamed fly I call a Red Tag.  

Contributions:

Courtney William's 1949 edition of "A Dictionary of Trout Flies" attributes the Red Tag to a Mr Flynn of Worcestershire, who called it the Worcester Gem and fished it on the Teme.  He gave it to F.M. Walbran who introduced it to the Yorkshire grayling rivers in 1878 and gave it the name Red Tag.   In John Roberts' 1995 edition of "Illustrated Dictionary of Trout Flies" he says the same thing.

It's the go-to dry fly in Tasmania.  It's probably the best known grayling pattern and I depend
on it in winter, especially with a gold bead head.  Bob

Hans Weilenmann:  In the German speaking countries of  western Europe, most notably Germany and Austria, the Hexe ("Witch") is an established pattern.  Basically, it a Red Tag with a grizzly, not brown, collar hackle.  
 

Bob Wyatt:  The Hexe is basically the old Grey Hackle.  Bob.

Ed Gallop:  Not sure about the "old" Grey Hackle.  The Grey Hackle I know of is tied like a Red Tag but uses red hackle instead of red wool and has grizzle hackle instead of reddish brown.  This is a fine example of the evolution of flies through variations.  

The Witch fly patterns date back to the early 1900s when they were originated by H. A. Rolt .  In addition to the original Witch, there were the Silver Witch and Gold Witch.  

Roger Woolley tied one named the Grayling Witch that is pictured in Taff Price's "Fly Patterns, An International Guide".  It is tied as is a Red Tag but with red floss for the tag and the body is heavily palmered with pale blue dun hackle.   It resembles the well known Wooly Worm.  I do not know if there is a name connection but more likely it is a coincidence.  I believe the Wooly Worm gets it's name from the look alike caterpillar nicknamed Wooly Worm.  Ed.

Hermann Schlibi:  In "The Way of a Trout with a Fly" (1921), Skues wrote, "It is, I think, beyond dispute that trout are extremely sensitive to red and are greatly attracted by it. Witness the value of a red tag to a fly."  I don't know if Skues was thinking of the specific Red Tag pattern you mention, but it is an interesting observation nonetheless.  

Note by Ed Gallop:  I may have provided more information than needed, if that is possible.  However, I welcome any input you can provide on this subject.   Ed.