It is hard to make any conclusions from that single photo, built it appears to be reasonably consistent with a 1934 to 1936 Bowlus (realistically 1935 or 1936 since Bowlus did not produce his first trailer until November 1934), or possibly a 1936-39 Airstream Clipper. Off hand I lean more toward the Bowlus because of the rear windows (which have been modified). Side or front photos will allow it to be identified more posirively.
It definitely is not a 1947-49 Curtis Wright Clipper.
I should also mention that at least a couple non-Bowlus, non-Aistream riveted aluminum "copy cat" trailers from the 1930s are known to exist, so that also remains a possibility.
Wally Byam Caravan Club International Historian
Vintage Airstream Club Historian