Centinela Av: There are two streets named Centinela, NS and EW. They seem to continue each other, although that was not the original plan. The NS Centinela runs from Brentwood, and forms part of the boundary between Santa Monica and Los Angeles. During WWII, part of Centinela was closed to make room for expansion of Santa Monica Airport (Clover Field). This was supposed to be temporary, and the street would reopen at the war's end. Of course this never happened. So Centinela turns into Bundy Dr.
The street ends at Jefferson Blvd. It seems at one time that the plan was to extend the street south across the Hughes Airport property, up a canyon into Westchester. There are early maps showing a NS Centinela Av in Westchester. This street is now Emerson Av.; this street looks as if it had been planned as a major street: the houses have garages opening on an alley in back, rather than to the street, and the parking strip is wider than normal. (This only applies to the parts between 77th St and 80th Pl, and between Manchester Av and the end of the street; the part between 80th Pl and Manchester has no parking strip; in fact, the curbs are slanted.)
Sepulveda Bl: This was at one time state highway #158 and part of highway #60. In the San Fernando Valley, the road was originally named Saugus Av. between Chatsworth St and Dickens St. There is a small segment of Saugus Av left south of Dickens St. North of Chatsworth St it took over part of Brand Blvd, and a new roadway was built to meet San Fernando Road. The portion of this new roadway north of Rinaldi St is now the San Diego or Golden State Freeway, except for the portion between Roxford St and San Fernando Road.
South of Dickens St this was new construction over the Santa Monica mountains, going under Mulholland Drive in a tunnel, and continuing through Sepulveda Canyon.
The portion through West Los Angeles may be new construction, or it may have taken over another street; I don't yet know.
Through Culver City there was new construction to the point where Sepulveda Blvd meets Jefferson Blvd. The portion between the two meetings with Jefferson Blvd was originally named (and numbered as) Jefferson Blvd.
South from Culver City appears to be new construction. But somewhere in Westchester it started following the route of Arizona Drive into El Segundo. This seems to have been an existing roadway, because it is on a section line.
The portion in Manhattan Beach was probably named Camino Real originally, as the extension of Camino Real in Redondo Beach. In Hermosa Beach, the name changes to Pacific Coast Highway. The portion where the west side is in Hermosa Beach and the east side is in Manhattan Beach (between Boundary Pl and Tennyson St), the street bears the name Sepulveda Bl on one side and Pacific Coast Hwy on the other!