When a poster is unsigned it is often impossible to discover who designed it. Artists did not sign works for many reasons. Sometimes artists were employed by a printer or studio that wished to be acknowledged rather than their employee. Members of an organisation’s publicity department designed other posters, and in some cases these artists were not acknowledged. Occasionally artists did not want it known that they were undertaking commercial work. Until a poster can be attributed to an artist it will therefore be credited ‘Anonymous’.
Information summarised from: ‘Anonymous’, Poster Database, London Transport Museum