Jefferson Blvd The original segment of Jefferson Blvd was called Jefferson St, in the old part of Los Angeles, like other "presidential" street names Washington and Adams: originally streets, now boulevards. Heading west on Jefferson Blvd, the route has changed somewhat. At one time, around 1950, the name "Jefferson Blvd" turned left on what is now La Cienega Blvd (originally Moynier Lane), and then right on Rodeo Rd. Then it headed in a southwesterly direction, as at present. Where Jefferson and Sepulveda Blvds come together in Culver City, the continuation was Jefferson Blvd. Around 1955, this segment was renamed Sepulveda Blvd, and the addresses were renumbered to fit Sepulveda.
Florence Av is at the level of what would be 72nd Street in Los Angeles. The plan of the 1930s was to extend it westward across Inglewood, just north of Centinela Park, running along what is now Hyde Park Place, then Centinela Avenue. A curve was built connecting Centinela Av with Jefferson Blvd; for years this curve was named "Florence Av"; later the original section of Centinela Av was renamed "Mesmer Av" and the Florence Av curve named "Centinela Av". Then there was to be another extension running just south of the Pacific Electric tracks to Lincoln Blvd. (This route is now used for part of the Marina (CA-90) Freeway.) The extension across Inglewood was never constructed, but instead Florence Avenue was linked to Inglewood-Redondo Road. The portion of Inglewood-Redondo Rd between the eastern limits of Inglewood and Arbor Vitae St was then renamed Florence Av. (The rest of Inglewood-Redondo Rd was named Aviation Blvd.) Later, the section between Manchester Bl and Arbor Vitae St was changed to Aviation Bl.
Easterly, Florence Av was constructed across Huntington Park, Bell, and Bell Gardens. A bridge was built across Rio Hondo, to link with the extension of Clara St in Downey, with a diagonal connection from west of Garfield Av to Scout Av, the old portion of Florence Ave in Bell Gardens being renamed Florence Place.
In Downey, Clara St ended at what is now Downey Av. A connection was built to what is now Brookshire Av (formerly, Lexington-Gallatin Rd), to Easy Street. The whole roadway in Downey (Clara St, the connection, and Easy St) was renamed Florence Av.
Florence Av continues through Santa Fe Springs. East of Shoemaker Av, it continued on what is now Sundance Av. But later it was extended on a new roadway to the north to connect with Mills Av at Telegraph Road. (Mills Av continues to Whittier Blvd, but retains the name Mills Av.)
Manchester Av: Manchester Av in Los Angeles was added to the state highway system in 1933 as highway 174. In 1934 it was given the sign route number 10, which was changed to 42 around 1960 to avoid duplication with Interstate 10. It connected some previously existing streets (whose names I do not know). From Downey eastward it was built parallel to the Southern Pacific tracks to Anaheim, to connect with state highway 2 (US 101). The portion from the eastern city limits of Los Angeles and the Orange County line was named Firestone Blvd because of the Firestone tire plant in South Gate.
The portion in the city of Los Angeles would be known as 86th St, if it were a numbered street.
As the road was under construction, route 10 left it, first along Rosecrans Av eastward, then later along Orangethorpe Av. From sometime in the 1940s the portion from Lakewood Blvd to the junction with US 101 in Anaheim bore the signage Bypass US 101.
When the Santa Ana Freeway was built, it took over the alignment of Firestone-Manchester from Rosecrans Av eastward, except for two segments about a mile long each, in Buena Park and Anaheim. At first the freeway had grade crossings. Then gradually bridges were built over the existing roadway. This is one of the few urban "at grade" freeways, although at the time of the upgrade, the area was largely dairy farms.