I think you will find that these are all very close to metric lengths in units of one or 0.5. 1 3/4in = 4cm, 2in = 5cm, 2 3/4in = 7cm, 3 1/2in = 9cm , 4in = 10cm. GB were an early pioneer in using metric dimensions - but the English market was not ready for cm or mm so the sizes were presented in inches.
Hilary Stone, Bristol, England
> From: theonetrueBob@webtv.net (Bob Hanson)
> Date: Wed, 21 Jun 2006 10:17:32 -0600
> To: Classicrendezvous@bikelist.org
> Subject: [CR] GB strm extensions
>
> 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