I found this
https://www.govinfo.gov/app/details/CFR-1999-title29-vol1/CFR-1999-title29-vol1-sec98-510
But I don’t think that’s it.
... or maybe it is.
Also found
29 -98 is lat/lon of San Antonio.
Also found a link to this:
https://www.irs.gov/pub/irs-irbs/irb98-15.pdf