When this recording is circulated, Frank Poretta is often listed as playing Steve on it. Poretta played Steve in the first three performances of the run, with Bob Shaver taking over for the final performance. Listening to the recording and doing comparisons with other recordings featuring Poretta and Shaver leaves no question that Shaver, not Poretta, is Steve on this recording.
The origin of this recording is uncertain. It has sometimes been identified as being from a radio brocast, but that seems extremely unlikely as no radio announcer is ever heard on the recording, not even during the three-and-a-half minutes of applause at the end (followed by a brief time when we hear the audience talking among themselves as they leave after the applause has stopped). It seems extremely unlikely that it would have been heard this way if it had been broadcast on the radio. (Some versions of this recording have edited out most of the curtain call.)
It seems possible that the recording was made for a radio broadcast that never occurred. Other possibilities include that it was recorded either off of or directly through the squawk box (stage monitor) or that Marc Blitzstein or someone else arranged for microphones to be set up onstage to record the performance with no intention of releasing it commercially. It's also possible that it was recorded through the theatre's sound system as contemporary accounts of the production state that the production was amplified, but this seems a bit unlikely as few sound-system recordings seem to exist from this early in time. Most recordings from this period that get identified as sound-system recordings were recorded with the squawk box as the source. Theatres usually employed just a single microphone for the purpose of transmitting the performance to the dressing rooms and to the stage manager.