A copyright search found nothing ever registered or renewed by Nicholas Volpe. The original drawings were displayed at the Brown Derby restaurant in Hollywood and never sold. All of the drawings in the collection have a copyright notice of 1962, which is when the collection of reprints was sold for $3. The PD status of the images has been reviewed at the Commons:Village pump/Copyright.
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (70 years p.m.a.), Mainland China (50 years p.m.a., not Hong Kong or Macao), Germany (70 years p.m.a.), Mexico (100 years p.m.a.), Switzerland (70 years p.m.a.), and other countries with individual treaties.