It occurs in the piano part of Sleepy Lagoon (Harry James Orchestra). The context is as follows:
C | | | | | Am | D9 | | G7+3 / G96 / | G7+3 / G96 / | G7 / Em F#m° | G7
I don't think it's a misprint, as it comes up again as G9+3 a few bars later. Since this is big-band jazz, it isn't plausible to me that it indicates to the player not to omit the 3rd as they would never consider doing that anyway.