This is when someone comes in the with Paxman's hot take that Corbyn's manifesto actually wasn't that left wing, and was simply Milibandism on steroids.
Worth Corbyn arguably has moved the party to the right on immigration, and has convinced an extremely pro-EU party to adopt a more euroskeptic tone. Of course the right of the party would have done the same.
My knowledge of pre-'79 Labour politics is shaky but wasn't the 1974-76 Wilson Government pretty on the left, at least before the recession really hit hard.
Rhetorically, absolutely, and there were some major condensations to the left--holding the 1975 referendum being one. Remember Healey's infamously mis-quoted promise to tax the rich until the "pips squeaked." This was pursued somewhat before '76 but there were still a raft of spending cuts.
The great u-turn--Healey Mark II--came in September '76 with the IMF bailout.