Yes, the M77s were marked on the side of the barrel "Made in the 200th Year of American Liberty". Every gun Ruger made in 1976 was so marked.
Even though the serial number range shows 1976 it made not have actually been made in that year. If it was made and shipped later, then it would be correct to not have the marking on the barrel. Or, it could have been re-barrelled later with a factory barrel.