Since Cory's varsity jacket has 1956 on the back, he must have been referring to 1955 as "last baseball season." He mentioned that Sandy Koufax led the league in strikeouts. In fact, Koufax was a rookie in 1955, appeared in only a few games and struck out only 30 batters. He didn't become a dominating pitcher until 1961, when he led the National League in strikeouts for the first time.
A store sign had a phone number shown as 412-nnn-nnnn. In 1956 phone numbers would not have been all-numeric (instead, they would have been something like EXbrook 7-1234), nor would they have had area codes as Direct Distance Dialing was not yet commonly used and most local calls would have been dialed with only the 7-character phone number - hence no need to show an area code.