Ambroin is / was made by Ver. Isolatorenwerke A.G. Vienna. Austria
Composition - Phenol formaldehyde
Described in Roget's Dictionary of Electrical Terms of 1924 as A moulded insulating material prepared from Copal and Silicates
An old brand name for Pilkington Heat Strengthened Glass
Telenduron was made by Thomas de la Rue & Co., London. UK
Composition - Unknown
In the plastics field Thomas de la Rue acquired interests in Telenduron products in 1914 whereby battery boxes and electrical insulators were moulded from bituminous compounds....
....Major business growth by 1937 led to the Walthamstowe Avenue Works enjoying large investments in 1000 to 1500 ton presses moulding novolak phenolic resins, the number of hydraulic compression presses growing to 70. Phenolic cloth laminate 'Delaron' sheeting was produced for electrical and mechanical use. In collaboration with the British Post Office De La Rue Plastics Ltd. produced components and coloured telephone housings over many years.
Hugh Burrows has sent me a snippet of information gleaned from an ex telephone engineer
I had a chat with a friend who was a telephone engineer and asked him if he knew the composition of Telenduron. Unfortunately not, but he did reveal that the product was deemed less favoured to the porcelain insulators on account of it seemingly tending to degrade and pit. With the pot heads of this manufacture he noted that it was important to make sure that the sealing compound fully filled the voids, and that the screw cap was tightly sealed otherwise corrosion soon took place at the joint faces.
Mike Chalton of Ayrshire also has some notes on Telenduron as follows
Just going through some notes I have, which gives the manufacturer of similar/same material as Litholite Mouldings Ltd. First produced in 1913 for the Indian Post Office, then used on field telephone lines during the First World War. Used a mixture of bitumen, petroleum pitch and short length asbestos (gasp) fibres. Put into a heated mould under hydraulic pressure of 100 tons.... Date of manufacture on such insulators is shown so that they can be renewed after five years service ! (Conductivity crept up with age!) They found favour in areas where stone-throwing was a problem!
As a consequence, I am now able to offer an explaination to several letter codes, LE and LEA, as used on various Telenduron Pothead lids