As you can see, much of the usage was in the state of Michigan specifically.
What was the influence?
A set of quadruplets — Krystal, Kristine, Keith, and Kenneth — born to Kenneth and Ann Rosebush of Oakwood, Michigan, on January 10, 1951. They lived in hospital incubators for several weeks before being allowed to go home.
Photos of the K-named quads regularly appeared in the papers during the early 1950s.
It’s hard to tell whether they had any influence on the names Keith and Kenneth, which were already on the rise in the early 1950s, but it does look like the name Kristine (which was sometimes misspelled Kristene in the papers) was affected:
1953: 1247 baby girls named Kristine
113 (9.0%) in MI
1952: 1885 baby girls named Kristine
206 (10.9%) in MI
1951: 1755 baby girls named Kristine
186 (10.6%) in MI
1950: 1247 baby girls named Kristine
110 (8.8%) in MI
1949: 1174 baby girls named Kristine
94 (8.0%) in MI
The Rosebush family also included four older children, all girls, named Dorothy (Dottie), Jacquelyn, Barbara, and Joann.
At a time when Kelly was bounding up the baby name charts, we see the debut (and quick rise) of the very similar Keely:
1960: 118 baby girls named Keely
1959: 119 baby girls named Keely
1958: 84 baby girls named Keely
1957: 7 baby girls named Keely [debut]
Keely debuted the year Virginia-born jazz vocalist Keely Smith had her first big solo hit, “I Wish You Love.” The next year, she and her duet partner/husband Louis Prima scored another hit with the song “That Old Black Magic.” In fact, the song won ‘Best Performance by a Vocal Group or Chorus’ at the very first Grammy Awards, in May of 1959.
Keely Smith, born Dorothy Jacqueline Keely, had spent much of the ’50s performing in Vegas with Prima. He had originally wanted to call her Dottie Mae Smith (Smith being her stepfather’s name) but, as she later said: “I was no Dottie Mae.” They settled on using her Irish surname as her first name instead. (The surname means “descendant of Caollaidhe,” with “Caollaidhe” being a male personal name derived from caol, meaning “slender.”)
Which name do you prefer, Keely or Kelly?
Hanks, Patrick. (Ed.) Dictionary of American Family Names. New York: Oxford University Press, 2003.