OK, I hope this link works... http://www.imdb.com/search/keyword?keywords=sequel&sort=release_date,asc&mode=simple&page=21&ref_=kw_nxt
It should take you to the 21st page of the 8744 sequels that have been made of movies.
That page is where the sound era begins (1927) and there are already 1,000 sequel on the list.
The first titles I recognized in 1927/28 were "Steamboat Willie" and "Tarzan."
Huckleberry Finn - 1931 a sequel to Tom Sawyer
Charlie Chan Carries On - 1931 (the first sequel of many)
Bride of Frankenstein - 1935 (the first sequel of many)
Dracula's Daughter - 1936 (the first sequel of many)
Hopalong Cassidy Returns - 1936 (the first sequel of many)
After the Thin Man - 1936 (the first sequel of many)
Frankenstein Meets the Wolf Man - 1943
Road to Bali - 1952 (the first sequel of many)
Son of Flubber - 1963
Zatoichi Meets Yojimbo - 1970 (two samurai universes joined)
Airport 1975 - 1974 (the first sequel of many)
The Godfather: Part II - 1974 (the first sequel of 2)
French Connection II - 1975
The Return of the Pink Panther - 1975 (the first sequel of many)
I didn't notice many sequel titles with "2" or "II" or even "part II" or "part 2" appended until "Godfather: Part II." (There were a few others but mostly foreign films or documentaries and who cares about those?)
I stopped looking before I got to the Rambo and Rocky sequels.