Okay, I finally broke down and looked it up :) Here is a list of Ron Howard's work between "Andy Griffith" and "Happy Days". BTW "Shootist" was in 1976 after "Happy Days" had already started.
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TV:
"Waltons, The" (1972) playing "Seth" in episode: "The Gift" (episode # 2.18) 24 January 1974
"M*A*S*H" (1972) playing "Private Wendell Peterson" (as Ronny Howard) in episode: "Sometimes You Hear the Bullet" (episode # 1.17) 28 January 1973
"Bonanza" (1959) playing "Ted Hoag" (as Ronny Howard) in episode: "The Initiation" (episode # 14.3) 26 September 1972
"Bold Ones: The New Doctors, The" (1969) playing "Cory Merlino" (as Ronny Howard) in episode: "Discovery at Fourteen" (episode # 3.11) 5 March 1972
"Love, American Style" (1969) playing "Richie Cunningham" in episode: "Love and Happy Days" (episode # 3.22a) 25 February 1972
"This Is Your Life" (1971) playing "Himself" in episode: "Andy Griffith" 25 March 1971
"Gunsmoke" (1955) playing "Jamie" (as Ronny Howard) in episode: "Charlie Noon" (episode # 15.7) 3 November 1969
"Lancer" (1968) playing "Turk Caudle" (as Ronny Howard) in episode: "The Measure of a Man" (episode # 1.25) 8 April 1969
"Daniel Boone" (1964) playing "Luke" (as Ronny Howard) in episode: "A Man Before His Time" (episode # 5.23) 3 April 1969
"Land of the Giants" (1968) playing "Jodar" (as Ronny Howard) in episode: "Genius at Work" (episode # 1.21) 9 March 1969
"Lancer" (1968) playing "Willy" (as Ronny Howard) in episode: "Jelly" (episode # 1.8) 19 November 1968
"F.B.I., The" (1965) playing "Jess Orkin" (as Ronny Howard) in episode: "The Runaways" (episode # 4.4) 13 October 1968
Film:
"Happy Days" (1974) TV Series .... Richard 'Richie' Cunningham (1974-1980)
... aka "Happy Days Again" (1974) (USA: syndication title)
Locusts (1974) (TV) .... Donny Fletcher
Spikes Gang, The (1974) .... Les Richter
Migrants, The (1974) (TV) .... Lyle Barlow
Happy Mother's Day, Love George (1973) .... Johnny
... aka Run, Stranger, Run (1973) (USA: TV title)
American Graffiti (1973) (as Ronny Howard) .... Steve Bolander
Wild Country, The (1971) (as Ronny Howard) .... Virgil
... aka Newcomers, The (1971)
"Smith Family, The" (1971) TV Series .... Bob Smith
Smoke (1970) (TV) .... Chris
Old Paint (1969)