letterboard

noun3

1. •
   A signboard for lettering or text.

   • So reads the new slogan painted on the letterboards of New York City omnibuses. — 1941
2. •
   A board marked with letters of the alphabet, used for communication by the disabled.

   • One of the patients, Leonard L., when he heard of L-DOPA, rapped on his letterboard with mixed enthusiasm and irony, 'Dopamine is Resurrectamine. Cotzias is the Chemical Messiah.' — 1973
3. •
   (historical) A board on which matter in type is placed for storage or manipulation.