W.B.lbl is correct, though, and I've switched the tracks. From Oct. 1949, Decca marked only its B sides with symbols like the diamond that indicated the pressing plant: http://www.friktech.com/labels/DeccaLabelography.pdf
The 'diamond' symbol after the matrix number would have actually indicated a designated 'B' side. Hence, the duet between Mr. Kaye and Jane Wyman would have been the plug.