House on Carroll Street barroom code

Kelly McGillis is Emily Crain. Jeff Daniels is FBI Special Agent Mike Cochran. Kenneth Welsh is his partner Joe Hackett. Edgar Allan Poe, the first murder mystery writer, was born in Boston, Massachusetts. He  lived much of his life in Baltimore, Maryland and he died there. 

Al Kaline was a Baseball Hall of Fame right-fielder from Baltimore who played his entire Major League career with the Detroit Tigers. He was known as one of the best right-fielders and overall players to ever take the field. His fellow players sometimes called him by his famous number, "Six."

On a scorecard, defensive positions are indicated by the numbers 1-9. Right field is 9. Shortstop is 6 (an upside down 9). The House on Carroll Street is set in 1951, two years before the Tigers signed Kaline straight out of high school  to play for the Major League club. The "shortstop" joke wasn't necessarily intended as a reference to a "future" All-Star by using a clever way to evoke his number with plausible deniability but if you know his number you know his name. If you know his name and number you can play a "name-that-Tiger" game with his number and the scorecard number for the shortstop.       

It might also be useful to know in the context of The House on Carroll Street and Fuhrman's role as an LAPD cop, that Shortstop is the name of a cop bar in Los Angeles.