The only question I would ask here - and this is really a question as I really don't know - is this...That's what I thought before I posted it and got some feedback. It is absolutely a 7-stroke ruff (with a drag lead in). To clarify, ruffs can be of any measured length equal to that of any roll, the difference is alternating single strokes versus alternating double strokes. They will have similair notation as well and thus need additional information to differentiate the two variants.
Here's some additional reference material, from the Buddy Rich book "Modern Interpretation of Snare Drum Rudiments" published in 1942, three years before Wilcoxon's The AAD.
From Pg. 16, Lesson 4 description: "All short Single Stroke Rolls are known as Ruffs."
Here is the lesson for the 7 Stroke Ruff page 24:
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Here is the lesson for the 7 Stroke Roll page 25:
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The musical notation for the ruff and roll is the same! The differentiation comes from the additional sticking/fingering information placed below the rhythm.
I see that Buddy's book was published in '42, but was his "Modern Interpretation of Snare Drum Rudiments" considered rudimental canon back then - I get it existed, but had its "Modern Interpretations" been as adapted into the mainstream of rudimental writing as you are suggesting.
I started studying in 1965 - and even by that time, I don't think it had. I can't remember seeing anyplace that what is described at the end of Lesson 19 - an 1/8th with two slashes - to mean to possibly play 6 single strokes.
Technically - notationally - that double slash notation (for all instruments) means "play four notes in the space of one" - in drumming we've added some variations, first and foremost in using that notation to mean either open or closed rolls - either 4 32nds or four multiple bounce strokes - OR - with added text to describe a seven stroke triplet based roll.
But I can't remember ever seeing any examples (besides these pages from "Modern Interpretations...) that suggest that playing a 7 stroke ruff as a common interpretation for that notation - short of there being specific text instructions tp play it that way.
Simply put - then and I believe today - that is simply not what that notation means - unless specifically instructed otherwise. And I believe is not something Wilcoxon would've intended with very clear spelling it out.
That would be far more unorthodox than writing a seven stroke as a hybrid rhythmic notation for a rhythmic effect ie: putting a drag before a 5 stroke roll. This sort of unusual rhythmic notation of a rudiment shows up in other Wilcoxon pieces and is quite common in Pratt's solos as well.
But IMO the last two bars of Lesson 19 are just wrong - you would not a 7 stoke ruff in that manner - not without specific clarification. And maybe not even then considering that ruff strokes are grace notes - not measured notes... generally speaking.