The Universal make-up department submitted two alien designs for consideration by the studio executives. The design that was rejected was saved and then later used as the Mutant in Universal's This Island Earth (1955).
According to a magazine article the "bubble" effect seen when the audience is seeing things from the alien's POV was achieved by actually blowing a specially formulated "tough" bubble around the camera lens. These shots were kept short since the bubbles only lasted a brief time.
Typing the helicopter's number into most web search engines will produce its registration and history (last checked in 2013). Although the copter is shown carrying three people in the film, it was registered as a two seat vehicle.
According to the tear-off calendar on the wall in the sheriff's office, around 50:00, the climactic day of the movie was "February 11 Saturday". February 11 was one of the actual days of filming, in 1953; however, in that year it fell on a Wednesday. Presumably, the set dresser used an old prop calendar for 1950 or 1939.