I’ve noticed that in my ACS data for 1970 (1970 1% metro fm1 and fm2) I only find 120 distinct values for metro area. I didn’t select any special cases for extraction. What might be causing this?
It seems that there should be more MSAs based on this web site:
Due to confidentiality requirements, in 1970, Standard Metropolitan Statistical Areas (SMSA) are identified in METAREA only if they contained 250,000+ residents. The discrepancy between the total number of SMSAs and the number of SMSAs identified in the IPUMS microdata reflects the exclusion of metro areas that fail to meet this population threshold.
Hope this helps.