I think you are correct.
From what I have read, Ruger did mark every gun they made in 1976 with that statement. Mine is a blued Security Six with six inch barrel, but it isn't on the barrel, it's on the left side of the frame, slightly above and behind the trigger. A search of the serial number of mine said it was made in 1977, though.
Another search I did tonight said Rugers with that "Liberty" mark are usually worth about $75 more than comparable models without it.