I don't think that is a valid comparision. The conditions for membership of the SM and CU when the UK joined the EEC were very different to that appertaining to membership now.Wrong.
EU did not exist then.
What existed then was, what brexiters call “free trading area” or “common market “, which they claim , they were happy to stay in before brexit.
What they do not admit, is that by leaving SM and CU , we left what they wanted to stay inside of .
That is just idiotic.
I daresay that if the UK could have the EEC conditions now we would still be a member, but the EU superstate project has moved on and, quite understandably, these conditions are no longer available.