Geographic Summary Levels but not ACS

This question refers to the 1980 Census, but am looking for some experts from long ago. Excuse the high-jacking of the ACS group for this, but the ACS is an outgrowth of the census.

Someone wanted data for an American Indian reservation for 1980. This doesn't look to be a summary level in 1980 even though I can find a published census volume that gives me the data he needs:

www2.census.gov/.../1980censusofpopu80211uns_bw.pdf

Does anyone have an explanation for this?