Nat Fleischer was them both and picked Corbett. Nat Fleischer was an infallible observer of everything. I can not contradict him.
So wait, Nat Fleischer was like Samuel Becket ever jumping through the space time continuum from boxer to boxer, winning fights until eventually he finds his way home. To be fair if he actually was both of those boxers, he knew which version of him was best, surely?
I think Tom Sharkey proved that a pressure fighter could throw Corbett off his game. Marciano is arguably the best there ever was at executing that type of fight against that type of fighter.
Sharkey fought a past his prime Corbett ... the best Corbett was 1891-1892 ... still , an interesting fight based on ring size and rules ...
I suppose the argument could be made that if Corbett was in front after 23rds against Jeffries and, bearing in mind this was a past prime Corbett, he would stay one step ahead of Marciano. Jeffries though , from news reads, was more reactive than pro-active. I feel Marciano would be much more direct and consistant with his attack . Marciano would initiate the exchanges at every opportunity,ie whenever he was in range of the fleet footed Corbett. He would not be too concerned where his artillery landed , just as long as he was connecting with shots. Whether that would result in him cornering Corbett ,after slowing him with body shots ,and applying the finish is the question? If you met Rocky head to head he eventually got you, but the clever ,cagey guys stuck around, Lastarza lost a split 10rd dec and ,went into the 11th the 2nd time. Charles went 15 rds and, into the 8th in the return. Old Jersey Joe went to the 13th. Tiger Ted, a defensive minded spoiler ,went the distance twice .:think Corbett might not have hit like Charles, or Walcott but he was probably more mobile than anyone Marciano faced.. This might be a better fight than most posters envisage.
10 or 12 you could pick Corbett. Corbett was a boxer mover type like LaStarza. I think Corrbett was a lot better than LaStarza who many claim beat Marciano in the first match, and was in the lead until Marciano fouled him with a mean hook on the break, and a low blow. But you said 20 rounds. I think Marciano had 20 round stamina. He would stop Corbett late.