I had an interesting conversation with a reputable manufacturer and fellow member of the NFPA 99 committee today about this topic...So that narrows it down to a handful of people anyway.  It relates to pigtails and other materials used in central supply systems.  The requirement in question is below from the 2018 edition:

5.1.3.5.4* Materials. Materials used in central supply systems shall meet the following requirements:

(1) In those portions of systems intended to handle oxygen at gauge pressures greater than 2413 kPa (350 psi), interconnecting hose shall contain no polymeric materials.  (For Reference Only)

(2) In those portions of systems intended to handle oxygen or nitrous oxide material, construction shall be compatible with oxygen under the temperatures and pressures to which the components can be exposed in the containment

and use of oxygen, nitrous oxide, mixtures of these gases, or mixtures containing more than 23.5 percent oxygen.

This requirement has been the same since the 2012 edition.  There is a twist though for those that are textualists when reading the code.  The wording changed from the following in the 2005 edition.

5.1.3.4.4* Materials used in central supply systems shall meet the following requirements:

(1) In those portions of systems intended to handle oxygen at

gauge pressures greater than 2070 kPa (300 psi), interconnecting

hose shall contain no polymeric materials. (For Reference Only)

(2) In those portions of systems intended to handle oxygen or nitrous oxide at gauge pressures of less than 2070 kPa (300 psi), material construction shall be compatible with oxygen under the temperatures and pressures to which the components can be exposed in the containment and use of oxygen, nitrous oxide, mixtures of these gases, or mixtures containing more than 23.5 percent oxygen.

You'll notice paragragh (1) changed from 300 psi to 350 psi.  That makes sense so that we can use these materials for flex connectors used with Dwyers.  (I believe!)

But the change to (2) I do not recall during the committee meetings and have overlooked it since the older editions.  It looks to me like the two requirements in the 2005 edition appear to go hand and hand.  One is for materials over 300 psi and one requirement for below 300 psi, which would basically mean that all materials below 300 psi would have to be cleaned for oxygen service.

And that makes sense to me, but does the 2018 requirement mean that we cannot use polymeric (teflon) coated pigtails for nitrous oxide systems?

I have a feeling that this was an editorial change that was not picked up, but I'm hoping someone can shed light on this if they have a reason why this might make sense as written.  A textualist might read it verbatim and not allow polymeric pigtails.

If there is no good reason, we might want to further discuss precedent a bit on this one because there are a boatload of these flex connectors being used with N2O manifolds out there.

There's always the 2024 edition to make the change if needed!  :)