If anything we should consider the No. 19 the real oddball, as I discussed upthread when I found mine, because it's not 19" OAL and doesn't fit the numbering/sizing scheme we can derive empirically from found examples. Why make a nominally 9" adjustable of the same style and construction and name it No. 19 if you're making a nominally 5" adjustable and naming it No. 5?! In other words, why isn't the 5-incher No. 15? And, in a production period we can reasonably assume is different from that ad you just posted, and other similar ads I have seen, they clearly made a nominally 7" adjustable of the same style and construction and named it No. 7, a 9" adjustable and named it No. 9, and an 11" adjustable and named it No. 11. The shift or inconsistency is maddening, and there are no corollaries for the No. 19 in other sizes. In other words, there are no No. 15, No. 17, or No. 111, where the 1 could be viewed as a model prefix and the second number as a nominal OAL.
Even though we have references for the No. 19, it's more puzzling, in my opinion, than the No. 5, No. 7, No. 9, and now, No. 11. They all make perfect sense, regardless of no references.