That's a good question BB. I don't have one.
But to help in the hunt, what about some evidence that they were offered for sale, so may have existed:
Looking in a few handy catalogues I find:
Marples 1928 and 1938: Ordinary (non-razee) jack planes - 14" to 17" long - all the same price. [The price depended on the width, and on extras such as parallel iron, striking button, closed handle, but you could choose your own length at no extra price. Whether your local toolmonger stocked a 14" plane was another issue - I suspect not!]
But they were not listed everywhere. For example, from another large vendor with a wide range - Skelton 1927 - non-razee jack planes - 16" to 18" only; choice of widths. "Jack Planes for Technical Schools" - 14" long, choice of width. Melhuish, 1925 - Technical Jack, 14" yes; but ordinary jack planes - 17" long only, whatever the width.