Paul Willen says self-amortizing mortgages were abundant before the 1930s

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.