Just looking through one of Chuck Schmidt's copies of a Ron Kitching Catalogue from 1963... Listed under GB alloy stems (all in inches) is: 13/4, 2, 23/4, 31/2, 4
My original 1960 Holdsworthy "Cyclists' Aids" booklet shows the Hiduminiun (alloy) stems in 13/4, 23/4, 31/2. This catalogue does, however, also list several GB models in steel, with many more variations in length, and these have even more peculiar measurements listed - such as 23/8, 35/8, and these also ranged to even longer extensions than were ever available from them (53/8, 53/4) in alloy variations.
With the less than standard lengths offered, in either material, perhaps GB was really not very concerned with presenting precise incremental increases which would fit neatly into a standardized chart of either metric or British sizing... to assist our comparisons across differing marques.
If Hilary Stone should happen to peruse this thread, I suspect he would have a great deal more info to offer on the subject.
Bob Hanson, Albuquerque, New Mexico, USA