Just for posterity, I realized that system_profiler and /sys do not provide exactly the same representation for the same information. For comparison:
system_profiler: Boot ROM Version: 4.8.9f4 Serial Number: YM5306NSTYX But: $ hexdump -C "/sys/firmware/devicetree/base/rom@ff800000/boot-rom@fff00000/BootROM-version" 00000000 24 30 30 30 34 2e 38 39 66 34 00 |$0004.89f4.| 0000000b $ hexdump -C "/sys/firmware/devicetree/base/serial-number" 00000000 54 59 58 00 00 00 00 00 00 00 00 00 00 59 4d 35 |TYX..........YM5| 00000010 33 30 36 4e 53 54 59 58 00 00 00 00 00 00 00 00 |306NSTYX........| 00000020 00 00 00 00 00 00 00 00 00 00 00 |...........| 0000002b
