Best Picture winner "The French Connection" (1971) got an official sequel in 1975, with Gene Hackman reprising his Oscar-winning role as Popeye Doyle, which is a pretty damn good movie, and underrated, with another terrific performance. But there is also a sort of quasi-sequel that came out between those two films,…

I always think of the 1998 film "Twilight" (absolutely nothing to do
with those teeny vampire movies) as an unofficial/unnamed sequel to
"Harper" (1966) and "The Drowning Pool" (1975).