He sends me the following table:
It has long been "established" that self-amortizing mortgages were rare before the existence of the Home Owners Loan Corporation, whereas this source suggests they made up 40 percent of loan originations between 1925-1929. What this table doesn't tell us is how long the amortization period was. So the importance of the HOLC may have been the establishment of long-term self-amortizing mortgages.
I would love to get actual mortgage contracts with their terms from the 1920s.