period piece vs souvenir

period piece

noun
  • A work of art or drama which evokes a particular historical era. 

  • A work which evokes the era in which it was produced, but may not have any other especial features. 

souvenir

noun
  • An item of sentimental value, that is given or kept to remember an event or location. 

verb
  • To take (something) as a souvenir, especially illicitly, for example during wartime. 

How often have the words period piece and souvenir occurred in a corpus of books? (source: Google Ngram Viewer )