Spiling sticks, joggle sticks and a “digital” rule

[Toolmaker51]

What I gather [pix of textbook not legible] and a particular contour/ notch pattern not a requirement; it seems to me a pointy version of a French Curve template. With enough such templates, allows any [nearly] set of points to form a curve........often with frustrating amount of trial and error.
The notches are unique in contour, so positioning is more accurate and distinct than a straight taper. Follows that picking up angular orientation better than a scale could, with need to record many deviations from an abstract datum. Main thing, the point represents actual contour, compared to "kerf" created by tracing a pencil tip.

Think that's what I thought while writing. Only one thing is sure; never seen or heard the like of, in any of it's names! So I'm googling it; one serious rabbit hole.

[Philip Davies]

What I gather [pix of textbook not legible] and a particular contour/ notch pattern not a requirement; it seems to me a pointy version of a French Curve template. With enough such templates, allows any [nearly] set of points to form a curve........often with frustrating amount of trial and error.
The notches are unique in contour, so positioning is more accurate and distinct than a straight taper. Follows that picking up angular orientation better than a scale could, with need to record many deviations from an abstract datum. Main thing, the point represents actual contour, compared to "kerf" created by tracing a pencil tip.

Think that's what I thought while writing. Only one thing is sure; never seen or heard the like of, in any of it's names! So I'm googling it; one serious rabbit hole.

Thanks, I am glad you were intrigued. If you wish, I’ll have another attempt at reproducing the text.