Respect the past, but build for the future. The principles of clear impulse lines, accessible valves, and robust mounting that RP 550 taught are timeless. The medium—pneumatic vs. electronic—has simply evolved.
Organizations like IHS Markit (now part of S&P Global) or Techstreet sometimes retain historical copies for sale. You can purchase a digital scan of the original RP 550 (usually dated 1985 or earlier). Expect to pay $150–$300 for a legal PDF. api rp 550 pdf