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This remarkable photo, part of a photoset of Chicago pictures I acquired from eBay member "nicepictures" (who graciously gave permission to post it here), clearly shows something which a number of us who lived through that era knew -- that on many weekdays during the 1960's and 1970's, the Cubs often closed the upper deck entirely, since crowds were small enough to all fit downstairs. Lower grandstand seats (what we now know as terrace boxes and terrace reserved) were unreserved, so you could sit wherever you wanted, first come first served, and the Cubs didn't have to assign ushers (they weren't called "security" in those days) to the upper deck.
Click on the photo and examine it carefully; then go after the jump to find out when it was taken. If you want to guess yourself, don't click on the jump to find out the answer before you make your best guess!

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The two identifiable Cubs in this photo are Rick Monday, playing center field and Jose Cardenal, playing left. That narrows it down to 1972-76, although I was given a clue by the photo itself; it was dated 1975.
That narrowed it down to six possible games, because if you look at the third-base coach, he's obviously wearing a Dodger uniform, and in those days, the West division teams visited Wrigley only six times a year. The first Dodger series was a weekend, May 30, May 31 and June 1 -- the crowds for that series were 21,197, 21,344 and 31,325, all too large to have the upper deck closed, plus, the Cubs didn't close the upper deck on weekends (at that time, there were no season tickets in the upper deck; in fact, there were likely fewer than 3,000 season tickets in all).
So it had to be the series of August 18, 19 and 21 (August 20, 1975 was a rainout, rescheduled for the next day). But which game?
Your next clue is the little mini-scoreboard visible on the facade of the upper deck. All it showed was the batter number, balls, strikes and outs. If you are wondering why there were two digits allowed for "strikes", it's because that board was also used for Bears games -- it showed, left to right, the Bears score, the quarter, down, the yard line, and the visitors score. You can also see the football press box underneath that scoreboard; it was rarely used during the baseball season.
The Dodgers' #34 in 1975 was worn by Lee Lacy, an outfielder of middling talent who had a couple of decent years with the Pirates in the early '80s. I checked to see if he had flied to center field in that series. Sure enough, he did so twice in the first game of the series, on Monday, August 18, 1975. But which flyout was it? One was in the fifth inning, the other the eighth. Both were the third out of the inning, as the board shows.
Your last clue is the fact that a Cub reliever is warming up. In the 1970's, virtually no manager would have had a relief pitcher warming up in the fifth inning of a 3-1 game (Steve Stone, who was the Cub starter that day, had given up a pair of homers in the fifth inning to produce that 3-1 deficit). But in the eighth? Sure, a reliever likely would have been getting loose, and in fact, Tom Dettore (whose biggest infamy came the next season, on April 14, 1976, when he gave up a homer to Dave Kingman, then playing for the Mets, which is believed to be the longest HR ever hit at Wrigley) was brought into the game in the ninth inning after Gene Hiser pinch-hit for Stone. It's not clear, but it probably is Dettore warming up in the pen.
The Cubs wound up losing the game 3-1, despite getting the first two runners on base in the bottom of the ninth (sound familiar?). This image, frozen in time, was the last out of the top of the eighth inning at Wrigley Field on Monday, August 18, 1975. Attendance on that cloudy day was 14,383. The 1975 Cubs actually had a pretty decent offense -- they tied for third in the NL with 712 runs. But their pitching staff doomed them to a 75-87 record; they allowed 827 runs, 88 more than anyone else in the National League.