When many in the modern Golden Dawn community first encountered the 5=6 sash worn by W.B. Yeats, which is on display at the National Library of Ireland, it presented what appeared like a very unusual design, with some suggesting that Yeats added symbols of his own. Even today it is often called the "Yeats sash," as if it was entirely unique to him.
Yeats' 5=6 sash (© National Library of Ireland)
The reality, however, is that this sash design is actually the official one, and was what all original Order members used, not the typical version we see all over the internet today. For example, here is Aleister Crowley's one:
Aleister Crowley's 5=6 sash
In case one might think this is a fluke, there are several other surviving sashes from original Order members, including ones attributed to A.E. Waite and W.A. Ayton, dated to c. 1892, the year when the Inner Order was formally established. There is even one from around 1920. All of these bear the exact same design as the one Yeats wore.
Furthermore, the original GD Portal ritual (which differs greatly to the SM version popularised by Regardie's book) describes the sash as follows:
"... white bordered with gold with the emblems of the 5=6 grade embroidered in gold thereon, and also those of the 24th, 25th, and 26th paths attributed to this grade."
While the above description does not make it clear what these "emblems" are, it does show that they are all gold, whereas the popular rendition of the 5=6 sash employs a red cross, red 5 and 6, and no path numbers at all (which breaks the trend set by the Outer Order sashes, where the path numbers are all employed on the sash).
So where did the design for the more familiar version come from? It was part of Westcott's original designs for the Inner Order sashes, in a document entitled Second Order Insignia, dated to 22 June, 1892.
In this document, Westcott draws designs for the 5=6, 6=5, and 7=4 sashes, as well as outlining plans for the jewels of the 5=6 sub-grades. Those who achieved the sub-grade of Adept Adeptus Minor (and only this sub-grade, not any of the preceding ones) were to receive "a different sash to the ordinary 5=6 members," which was to be born crosswise over the ordinary 5=6 sash.
Westcott then proceeds to describe this sash for Adept Adeptus Minor, and, lo and behold, it is the 5=6 sash we are all familiar with. Of particular note is the employment of the colour red for the cross and numbers of the grade, "to show the link with Geburah."
Adept Adeptus Minor sash design by Westcott
This sash, therefore, is not incorrect, per se, but it is designed, at least originally, for those who have fully completed the 5=6 sub-grade curriculum, and are, as it were, candidates for the 6=5 grade, attributed to Geburah. Before then, at the lower sub-grade levels, the use of red upon the sash could be seen as somewhat jumping the gun.
As for the provenance of the design used for ordinary 5=6 members, it is possible it was created by Mathers, who wrote the Portal ceremony, and who would have clearly understood the importance of adding the numbers of the paths. Alternatively, it may have been designed years previously by Westcott. After all, there were those who were nominal 5=6 members before Mathers wrote the 5=6 ceremony, so it is possible the sashes were in use earlier than 1892.