Looking at this photo, you'd think it would be almost impossible to figure out when it was taken. And you're right -- there aren't many clues, but there are enough to at least pinpoint the date, if not the inning.
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But that's not Boccabella -- it's Terry Humphrey, who was an Expos backup catcher from 1971-74. He played in only two Wrigley Field games that Cardenal also played in -- April 7, 1973 and the second game of a doubleheader on April 13, 1974 (I was actually at that doubleheader).
So which is it? It looks cold in the photo, and I can tell you from having been at the 1974 doubleheader that I remember it as being sunny (and 61 degrees, according to the boxscore).
Thus, this photo was taken on April 7, 1973, a game in which the Cubs blew a 2-0 sixth-inning lead but won 3-2 in 10 innings on a walkoff single by Ron Santo. It's impossible to tell which of Cardenal's at-bats this is; you can't see the pitcher or anyone else on the field except plate umpire Bill Williams and Expos SS Tim Foli, wearing that red-white-and-blue Expos beanie.