I found that almost every NCAA rulebook since 1928 can be found on the NWHOF site. I wanted to look up something about the history of minimum and/or mandated mat sizes. But I found something about the actual meaning of pin fall!
We, today, think of pin and fall as being equivalent terms. That's not how they thought about them years ago.
A fall was when the shoulders touched the mat. For it to be a termination of the match, it had to be pinned down. In other words, in a controlled fashion and for a specified length of time (2 seconds early on).
From the 1950 book as an example-
RULE 15-FALLS AND NEAR FALLS
1. Pin Falls.
Pin falls only shall count. (Flying or rolling falls shall not be
considered.) Any part of both shoulders held in contact wIth the
mat for an appreciable length of time constitutes a fall. (By an
"appreciable length of time" is meant the Referee's sIlent count
of two seconds. The two-second count shall start only after the
Referee is in such position that he knows positively that both of
the defensive contestant's shoulders are in contact wIth the mat,
after which shoulders must be held in continuous contact wIth
the mat for two seconds before a fall shall be awarded.)