The first 121 works issued in the first Penguin Classics series (plus No. 125) shared a cover design that had a narrow coloured border around a white panel containing the title, etc. and a more or less circular black and white line drawing - the 'Medallion' or 'Roundel'. These were published between 1946 and 1962.
As can be seen from the Medallion covers that have made it to the display here, most of the drawings do have the appearance of a medal or coin but from L32, The Four Gospels, onward, larger and different shaped pictures start to appear. I do not think colour was used except for L30, Scarlet and Black where the background of the roundel was black and red.
The border colour indicated the original language of the work, brown for Greek, green for French, red for Russian, and so on. This convention did survive for a time in the early Black Cover titles.
The first four or five titles were originally issued with a slightly different cover with the medallion lying on a much deeper coloured border along the lower edge of the book. Reprints used the later standard format.
The majority of these works continued in print for many years so can be found with later cover designs. They are also listed in the Penguin Classics series.