In Episode 35, Jeff was still wearing #4 --- at least in the barnstorming uniform he donned for the Lemo broadcast:


It's funny that the producers were soooooo careful with the pilot to make sure that nothing bore too close a resemblance to any real-life people or places --- and they picked an actual 1946 team member's number for Jeff's uniform!

:indians

Jack Conway was born on Tuesday, July 30, 1918, in Bryan, Texas. Conway was 23 years old when he broke into the big leagues on September 9, 1941, with the Cleveland Indians.

www.baseball-almanac.com/players/player.php?p=conwaja01

Jack stood 5' 11 1/2" tall and weighed in at 175 lbs. --- just about like Jeff! Unlike his television counterpart, he played college ball for the University of Texas. After a partial season with the Indians, he must have enlisted or been drafted and didn't return to the game until 1946. Jack spent two full seasons with the Indians, then appears on the roster of the New York Giants for a portion of the 1948 season. He is supposed to have played his last major league game on July 31st of that year. Injury, perhaps? One article refers to Jack as a second baseman.

:baseball

Check this out! It's a baseball signed by the 1947 Indians with Jack Conway's signature and "Jeff's" manager, Lou Boudreau, and his teammate, Bob Feller! Remember the episode where Jeff is sitting in the locker room signing a box full of balls and blowing on the ink so his signature will dry without smudging?
Bethany