sorry i did not include that one because is was suggestins as opposed to standards. I though the c-6.2.1 in included. Youa re correct it is related material. The law issue can be argued either direction but my understanding that the law says compliance with recommendations is required. Lawmaking by proxy. Which gave birth to the 23.5% rule/ proxy law.
Regarding visuals for scuba cylinders and frequency:
CGA P5 Suggestions for the Care of High Pressure Air Cylinders for Underwater Breathing
Which before anyone gets their hoses in knots - the above is not referenced by any CFRs and is not law. Further, they are suggestions not requirements.