It's almost impossible to know because no one here was watching every regular season game in the 1970s. And you have to watch regular season games on the regular to know who deserves DPOY, otherwise you just going off of stats and rep.
From what I've heard, no one had ever really tried to play defense hard until Russell entered the league, and he was giving full effort on defense from day 1 as well as having the height, athleticism, and bball IQ to be elite. So he probably wins it every single year from his rookie season until the end of his prime.
Even after Russell the average day-to-day defense was pretty weak, teams only tightened up in the crunch or for big rivalry games. Guards were never locking guys down like that, so I'd probably just be giving DPOY to the big man who was leading the NBA in blocks each year. I'm guessing Hayes, Thurmond, Gilmore, Kareem, and Walton would all be in the running for a couple of them.