A small promotional booklet published by L C Smith (& Corona) of Aldwych House, London on their programme of standard office machines.
The booklet explains the standard L C Smith typewriters and the many varied attachments that are available for them. Apart from platens in different hardness, also a variety of specialty platens. There must have been thousands made also of these special platens, but haven't seen any of these - perhaps not many survived.
Even though this is very much about the large standards, the booklet ends with the Corona - they are also the makers of The Personal Writing Machine!
The complete booklet about the L C Smith ball bearing Office Machine is available on The Archive.