According to this image, a certain Antonio Rodríguez Lesende replaced Roberto Rufino for a while as Di Sarli’s singer because of illness. And indeed, there is (just) one 1940 recording, ”Milonga del Centenario” with that unfamiliar voice, or at least unfamiliar to most dancers nowadays.
However, interestingly, Rodríguez Lesende was actually a famous artist back then, and Troilo’s first choice to become the emblematic singer of his orchestra rather than Francisco Fiorentino. Rodríguez Lesende declined the job, because he didn’t want to work nights (source: Michael Lavocah, Tango Masters: Aníbal Troilo) and seemed to have been overloaded with work in general. Now, nobody remembers his name.