I've been trying to use TMP community resources to establish as objectively as possible the origin of the confusion surrounding miniature scales. I invite anyone with objective evidence on the subject to add to the project. Evidence must be documents from the time or measurements of figures of the time, anecdotes and memories are not reliable evidence.
Here's what we've established so far.
Pre 1950
The earliest figures used for war gaming were a combination of following the scales associated with model railroad gauges (54mm is 1/32 the scale associated with I' gauge) and the mainly German tradition of making figures to a set height e.g. 30mm'. In both cases the millimeter designation refers to the overall height, sole to crown of the figure.
1950's
Thomas Millimeter height is sole to crown
Scruby Millimeter height is sole to crown
SAE- Millimeter height is sole to crown
Greenwood Millimeter height is sole to crown
In particular Scruby published an illustration comparing different size-scales in his 1968 catalog:
tabletoptalk.com/?p=686 so there is no question of that he understood the millimeter size to mean sole to crown.
1960's Here's where things begin to get murky
Hinton Hunt markets 20mm' figures which are about 23.5mm tall sole to crown. This can not be explained by measuring to the eye as that would still be 22mm. My best guess is they are meant to be HO/OO scale, a British model railroad convention of putting 1/76 scale trains on 1/87 scale track. 20mm figures are 1/87 HO' and were frequently marketed as'20mm/HO scale' so when Hinton Hunt went to market their figures they called them HO/20mm meaning 1/76 British HO. This is the size of Airfix plastic figures.
Early Minifigs follow Hinton Hunt's lead making 20mm' figures which are about 24mm sole to crown.
Jack Scruby responds by making matching figures and calling them 25mm, this is the origin of 25mm'
which will later morph into 28mm.
1970's
British and American convention coalesces around describing figures by actual millimeter height, Minifigs for example begins calling their figures 25mm'.
Modeling in epoxy, as opposed to carving in wax or solder becomes widespread. This exacerbates the tendency for figures to come out larger than the sculptor intended.
Fantasy figures become popular, many fantasy characters and creatures are larger or smaller than an average man making the size-scale' confusing to apply.
1980 First documented case we've found so far of describing figure by height to eye, advertisement for Viking Miniatures in April 'Military Modeling'.
Barrett scale system for describing figure compatibility published in The Courier' magazine uses measuring to the eye.
28mm' begins to replace 25mm', also new designation heroic X-mm' meaning bigger than other figures of that size-scale.