I started going around the same time, and I'm probably a lot lot younger than you
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I don't think we would have played with two central defenders in the early sixties.
I would say we operated a 3-3-4 formation, for the most part, with Greig the deeper of the midfield trio.
By the mid sixties, some teams were using 4 at the back ie 4-2-4 or as Ramsey's England 4-4-2.
Celtic used McNeill and Clark as a central defensive pairing, but I think that Clark was more there to cover McNeill who was not fast, and was often beaten easily on the deck.
McKinnon was fast, and not easily beaten on the deck, and therefore did not need the insurance of cover in the same way.
Having said that, I think at Wembley 67' Greig and McKinnon were a central defensive partnership.
Even with Davie White, I think Greig was the central Midfielder, with Penman, and Dave Smith either side of him.
I would say it was the very end of the sixties, start of the seventies, before we used two central defenders, as in Jackson/McKinnon, or Johnstone/Smith.
That is just my take on it.