Further digging reveals that it's related to https://en.wikipedia.org/wiki/Intel_Binary_Compatibility_Sta... (which I couldn't find the actual document of) and might explain where the original set of system call numbers in Linux came from. This also reminds me of the "CALL 5" interface in DOS which was used for CP/M compatibility.
Linux did have lcall7 and lcall27 compatability support
There is very little information available online about that --- section 2.11 here might be a better description:
https://tldp.org/LDP/lki/lki-2.html
Further digging reveals that it's related to https://en.wikipedia.org/wiki/Intel_Binary_Compatibility_Sta... (which I couldn't find the actual document of) and might explain where the original set of system call numbers in Linux came from. This also reminds me of the "CALL 5" interface in DOS which was used for CP/M compatibility.