Quarlo's battle helmet would later be reused as the helmet worn by the alien Mork (Robin Williams) in Mork & Mindy.
In The Outer Limits: The Official Companion, Harlan Ellison said that he was upset when the producers inexplicably changed Quarlo's serial number to serial letters. He called it stupid, and explained that you can't have an army with serial letters because there are fewer combinations. In fact, the producers were correct. Since there are 26 letters versus 10 digits, there would be 2.6 times more combinations per space using just letters versus using just numbers. For an 8 letter code like the one used in "Soldier," there would be over 208 billion combinations with letters versus only 100 million for numbers.
Along with Demon with a Glass Hand (1964), this is one of two episodes penned by Harlan Ellison that allegedly formed some of the basis of James Cameron's script for The Terminator (1984).
The alley in which Quarlo first appears in the 20th century was on the MGM backlot #2. It was a three-way conjunction between Eastside St., Brownstone St., and the main avenue through their New York section called Fifth Avenue.
The writer, Harlan Ellison, ended up suing the producers of The Terminator movie for copying his story of two time-traveling soldiers sent to the past who continue their battle. Ellison won the lawsuit.