A search was done for renewals in artwork for the years 1965 and 1966 There were no renewals for Gene Byrnes or Reg'lar Fellers. There's no evidence that this comic strip is still under 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.