The history of the various incarnations of Route 10 are a bit complex. Take a look at
http://www.cahighways.org/009-016.html#010 . Pasting a bit:
In 1934, Route 10 was signed along the routing from Jct. Route 3 (US 101A, later Route 1) south of Venice to Jct. US 101 at Santa Ana, via Manchester Avenue and Santa Ana Blvd. This routing was LRN 174 from US 101A (Route 1, Lincoln Blvd) in Los Angeles along Manchester and Firestone Blvds to US 101 in Norwalk. Sometime between 1934 and 1963, the routing was resigned as Route 42. Specifics are not available, but the guess is that the resignage occurred in the late 1950s in preparation for the interstate. (1956 and 1960 maps shows it as Route 10; the 1963 state map (pre-renumbering) shows it as Route 42). Before signage as US-101, the routing (signed as Route 10, but LRN 174) continued on down to Orangethorpe, and then across Orangethorpe past Route 101 (Spadra Road, at that time) and E through Atwood, until joining the old surface route equivalent to US 101 (LRN 2). Some maps show Route 10 ending at the junction with Route 18 (later renumbered as Route 14, but cosigned with US 91; LRN 175 and LRN 178). It appears that, by 1942, Route 10 was also signed as Bypass US 101.
On July 1, 1964, LRN 174 (at one time signed as Route 10), between US 101A and US 101 in Norwalk, was officially designated Route 42, and has since been deleted from the state highway system.
In 1963, the first segment of Route 10 (I-10, Santa Monica Freeway) was defined as "Route 1 in Santa Monica to Route 105 near Soto Street." (Route 105, at that time, was the portion of current US 101 from the San Bernardino to Santa Monica freeways)
In 1968, Chapter 385 changed the definitions of Route 105 ("from Route 5 to the junction of Route 110 (now part of Route 10) and US-101") and Route 110 ("from Route 105 to the junction of Routes 5 and 10") from their former stub routes in downtown, creating the present day I-105 routing. At this point, the definition of Route 10 was changed to "Route 1 in Santa Monica to Route 5 near Seventh Street in Los Angeles", and US 101 was changed to start at "Route 5 near Seventh Street in Los Angeles".
This segment is now From Route 1 in Santa Monica to Route 5 near Seventh Street in Los Angeles.
The San Bernardino Segment is as follows:
In 1963, this routing was defined as "Route 110 in Los Angeles to the Arizona state line at the Colorado River via the vicinity of Monterey Park, Pomona, Colton, Indio, and Shaver's Summit, and via Blythe, and includes that portion of the Colorado River highway bridge (near Ehrenburg, Arizona) which is within the State of California. The department may contract with the State of Arizona, for and on behalf of the State of California, for the maintenance of such bridge." Route 110 referred to a stub route downtown (in particular, the portion between the two segments of Route 10).
Some history of the East LA interchange, including the connection between the two segments of I-10, may be found in the discussion of US 101.
In 1968, the stub Route 105 and Route 110 were elimated, and the portion from Route 101 to Route 5 was transferred from former Route 110. This changed the routing to "(b) Route 101 near Mission Road in Los Angeles to Route 5. (c) Route 5 in Los Angeles to the Arizona state line at the Colorado River...", reflecting the slight discontinuity at Route 5.
In 1984, the two segments were combined, and the text about Arizona was removed, giving the definition of "(b) Route 101 near Mission Road in Los Angeles to the Arizona state line at the Colorado River via the vicinity of Monterey Park, Pomona, Colton, Indio, and Chiriaco Summit and via Blythe."