Shoulder roll bars were mandatory in 1954-55 by the SCTA. A head hoop came in 56 I believe and bracing after that.
Here's a Roadster in 1955 -- looks like no rollbar required. I doubt that RPU was a competitor -- they're a whole lot of hot rods there every year -- more than the number of competitors.
That roadster had a legal shoulder bar. Many were under the body lip. Jack Underwood and his father were there during that time helping other Compton guys and running their roadster and were talked of that ear often.