HUH? 645.5(D)(2) refers to the branch circuit that serves the receptacle, not the line cord. SO is not one of the wiring methods anyway.
We had to rip out all of the SO cord under the floor in the mid 70s. That is where I got my welder extension cord.
645.5(D)(2)The branch-circuit supply conductors to receptacles or field-wired equipment are in rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, electrical metallic tubing, electrical nonmetallic tubing, metal wireway, nonmetallic wireway, surface metal raceway with metal cover, nonmetallic surface raceway, flexible metal conduit, liquidtight flexible metal conduit, or liquidtight flexible nonmetallic conduit, Type MI cable, Type MC cable, or Type AC cable. These supply conductors shall be installed in accordance with the requirements of 300.11.
645.5(D)(5)Cables, other than those covered in (2) and those complying with (a), (b), and (c), shall be listed as Type DP cable having adequate fire-resistant characteristics suitable for use under raised floors of an information technology equipment room.
(a) Interconnecting cables enclosed in a raceway.
(b) Interconnecting cables listed with equipment manufactured prior to July 1, 1994, being installed with that equipment.
(c) Cable type designations Type TC (Article 336); Types CL2, CL3, and PLTC (Article 725); Type ITC (Article 727); Types NPLF and FPL (Article 760); Types OFC and OFN (Article 770); Types CM and MP (Article 800); and Type CATV (Article 820). These designations shall be permitted to have an additional letter P or R or G. Green, with one or more yellow stripes, insulated single conductor cables, 4 AWG and larger, marked “for use in cable trays” or “for CT use” shall be permitted for equipment grounding.
I doubt you will see much pre 1994 equipment these days. It was pretty much all gone when I retired in 96. There was a significant technology change in that timeframe. Disks went from the large format media to racks full of 3.5" drives similar to what is in a PC. Processors, controllers etc went from transistor logic to CMOS logic which allowed smaller boards and very much lower operating currents. That was the end of the "glass house" computer room as we knew it. That room full of walk in freezer sized boxes became a rack.
I lost interest in computers when they stopped having hydraulic pumps in them
