Good question. The Giants played 28 people, total, so it can't be they used all different people on O and D. It sounds like he had the same eleven basically playing both ways for 7 1/2 minutes and then sent in a different eleven. Maybe a football historian can enlighten us.
While baseball is little changed in rules and style of play (with the exception of the abomination of that stupid rule in the AL) football of 1939 would be virtually unrecognizable to today’s fan. In fact, football of 1939 has evolved in much the same way as warfare. Old ground-slogging infantry has been replaced by high-tech aerial assaults.
But what made it all possible was the evolution of rules regarding substitution of players. I found a nice summary of coolegiate rules but in 1939, the pros were similar:
From 1922 to 1940 all “players withdrawn during the first half may not return until the second half. Players withdrawn during the second half may not return to the game”. This includes injured players. The significant rule change in 1939 that still remains in today’s rules: is all players were required to wear “head protectors”. 1939 was the first year this rule came into being a mandatory rule.
Foot Ball Rule 5 section 1 governed substitutions in 1939. A substitute had to report to the UMPIRE before participating [5 yd. penalty for not reporting]. The substitute was prohibited from communicating with anyone other than the umpire until the ball was snapped. [15 yd. penalty if he communicated with anyone].
SUBSTITUTIONS:
1876—Fifteen players to a team and few if any substitutions.
1882—Replacements for disqualified or injured players.
1897—Substitutes may enter game at any time at discretion of captains.
1922—Players withdrawn during the first half may not return until the second half. Players withdrawn during the second half may not return to the game.
1941—Players may substitute at any time but may not be withdrawn or the outgoing player returned until at least one play has intervened. “Platoon” football was made possible. This is one of the affects of WW I and WWII - the military’s influence on the game and personnel.
1948—Unlimited substitution on change of team possession.
1953—Two-platoon abolished and players allowed to enter game only once in each quarter
1954-64—Changes each year toward more liberalized substitution and platoon football.
1965—Platoon football returns. Unlimited substitutions between periods and after a score.
1974—Substitutes must be in for one play and replaced players out for one play.
1993—Players who are bleeding or whose uniforms are saturated with blood must come out of the game until their return has been approved by medical personnel.
http://en.allexperts.com/q/College-Football-2792/2008/5/substitution-rules-1939.htm
In 1939 players were expected to play both ways. Think how different that made the game from today.