"... two steel rulers lie next to each other, coming together at a highly oblique angle. Along the inner edges of these rulers, a scale is marked out. The thinner the string, the further it can be slid into the wedge-shaped opening between the rulers, and this distance... indicates the thickness of the string." - Blüthner's 1888 instruction for the student of piano making
Of all the ways of measuring wire, the slip gauge is the most accurate of the "primitive" methods, which also include the slotted gauge plate and the hole plate. Not only is it more accurate than the other two, because it displays diameter in an infinitely-continuous manner (not in increments), but it is also much easier to make, requiring only one reference wire and a knowledge of the gauge system logic (at least for systems with regular increments, either logarithmic or arithmetic). This is probably why Blüthner called it "very useful" and "exceptionally well-suited to the measuring of strings for the piano", even though his discussion of "chordometers" covered everything from the primitive slot plate to the modern micrometer.
Here we see a slip gauge produced by the Viennese company Martin Miller & Sohn (photo from the Huber/Saitendrahtsysteme). The Miller firm began making piano wire in 1840, and according to Blüthner, their strings were highly regarded for strength and tuning stability. This gauge is more support for the theory that the slip gauge was the standard method of measuring wire among piano makers before the advent of the micrometer.
That some musical instrument manufacturers followed the advise of Thon is proven by another slip gauge, given to me by Christopher Clarke. The back is stamped with the date 1891. Judging by the diameter range it measures and the 4 1/2 octave compass, it was probably for a harp. Despite it's diminutive size, it is very accurate: one can easily see the difference between adjacent sizes of Rose piano wire.
This illustration shows how the strings found on a c.1820 piano by the Viennese builder Rausch (Rose/Law #111, page 123) would have appeared when sorted with a slip gauge constructed with Bleyer's "Nürnberg" proportions at a reference diameter of 0.635mm for gauge 1/0. The agreement is near-perfect. Only the heaviest gauge is so out-of-tolerance that it properly ought to be given the next designation. However, if Rausch had (or could get) no other wire at the moment, he would have obviously made do with what was on hand, though he would have marked the instrument following his normal "ideal" gauge distribution.