In the minutes from the British Telephone Technical Development Committee, Subscribers Apparatus Development Sub-Committee meetings it states that Diakon was not satisfactory for cases and and that nylon should be used.  As these minutes were about field trails, the history seems to fade......

Diakon, as the "Perspex" was called was used from 1959 until the end of 1962 and ABS was used from then on.  The Diakon telephones don't get faded and look s nice and shiny. The ABS ones aren't shiny, but fade and get "nicotine-looking".  A quick check apart from the date, as very early 1963 phones can be Diakon still, is to unscrew an earcap and look at the number under the cap, providing it matches and hasn't been changed.  The number 26 would be Diakon and number 26A would be ABS.

Be very careful of Diakon as it chips and breaks easily!

The ABS cases were THINNER than the Diakon ones, because they were stronger.  This made the moulding cheaper and lighter.  This gives another way of telling ABS from Diakon.  There were strengthening webs inside to restore the original thickness locally for rigidity, and in some instances to ensure a snug fit.  TMC had problems with these, since they tended to cause "sink marks" on the outside, so the webs were made very thin on later supplies, which solved the sinking feeling whilst serving the intended purposes adequately. 

Diakon was moulding material 12 and ABS was 17 - these will appear in the moulding marks.

I am not aware that Nylon was ever seriously used.  ABS was a very new material in 1962, so the degree of improvement it would bring was probably unknown.  The breakage rate of Diakon was still a SERIOUS problem in the mid-60s, as large numbers of Diakon 706s were still in use, and probably being "wiped up" (GPO factory repaired) using local stocks of Diakon mouldings...

Many thanks to Geoff Mawdling and Paul Ebling


BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: October 30, 2022