Originally the routing of US 99 via San Bernardino, circa 1926, was this: West on Colton Ave. (now Redlands Blvd.) to a broad curve west of Waterman that deposits the route on South E Street. It continued north on E Street to 3rd Street, where it turned west with south SSR 18 (NB 18 continued north on E St.). That continued past the Santa Fe depot to Mt. Vernon, at the south end of the bridge over the Santa Fe tracks; 99 turned north to cross the bridge (this was several years prior to the commissioning of US 395), while SSR 18 turned south onto Mt. Vernon toward Colton. At the north end of the bridge Mt. Vernon intersected 4th Street/US 66 West; US 99 turned west there as well, multiplexing with US 66 into Pasadena. At some point in time between 1926 and 1934 Business 66 was established; from west to east it diverged from mainline US 66 at Mt. Vernon, crossing the bridge south with US 99 before turning east with both south US 99 and north SSR 18 on 3rd. Street. It turned north on E Street with SSR 18, continuing north to Highland, where SSR 30 crossed and SSR 18 turned right with it. Business 66 continued north on E Street, which eventually turned NW and became Kendall Ave. US 66 east continued north on Mt. Vernon to Highland, where it turned northwest onto Cajon Blvd., the road up the south side of Cajon Pass, intersecting Business 66/Kendall Ave. at a point about 5 miles north.
The original SLR's for the area were equally convoluted. The "spine" SLR through the region was SLR 43, which followed SSR 18 throughout the region; a number of long-distance SLR's terminated at this legislative route. SLR 26 was US 99 south of the 3rd Street/E Street intersection, following US 99 all the way to the Mexican border at Calexico. SLR 31, which carried US 66 over Cajon Pass to Barstow, and US 91 (which was not extended south of Barstow until 1947) northeast from there to Nevada, began at the junction of Mt. Vernon and 3rd Street and was the Mt. Vernon RR bridge and US 66 east from there. SLR 9 was US 66, and originally US 99, extending west from SLR 31 on 4th Street toward Pasadena. SSR 30/Highland Ave. was SLR 190, and North E Street and Kendall Ave. (Biz 66) was SLR 191.
This arrangement didn't last long; the bridge over the Santa Ana River connecting Colton Ave. in the south reaches of San Bernardino with Valley Blvd. in central Colton was completed circa 1931; SLR 26 and US 99 were rerouted west along that alignment. South E Street from the rerouted US 99/SLR 26 became a realigned but unsigned SLR 43. 3rd Street, still carrying SSR 18, became an eastern extension of SLR 9, which terminated at 3rd and E Street at SLR 43 (SSR 18 still turned north there). The entirety of Mt. Vernon north of Valley Blvd. in Colton was reassigned to SLR 31; it still carried SSR 18 south of 3rd Street until US 395 was commissioned in 1934 to join it. US 99/SLR 26 continued west from Colton on Valley Blvd. into Ontario, where it was renamed Holt Ave.
Like with 4th street west of Mt. Vernon, 3rd Street past the original Santa Fe depot (now used by Metrolink & Amtrak) east of the Mt. Vernon bridge features portions of the original segmented concrete pavement that once carried US 99 & SSR 18. Actually, the east-west Valley Blvd. alignment that eventually became US 99/SLR 26 was in place as a county facility at the time US 99 was routed through central San Bernardino, with its east end at or near Mt. Vernon Ave. in Colton. The delay involved with realigning US 99 was due to the difficulty experienced in constructing bridges over the Santa Ana River; the riverbed consists of deep layers of sand and silt lying in scooped-out sandstone with no supporting bedrock to speak of. Bridge construction over such a "waterway" required placing concrete caissons deep into the sand and waiting for them to settle before constructing bridge piers upwards from them -- then letting the whole assembly settle even more before extending a bridge deck over the piers. This method has continued to this day; the I-10, I-215, and CA 210 bridges over the Santa Ana River were deployed in a similar manner. The original US 99 E Street bridge was the only exception; it was a relatively light "trestle" structure sitting atop the silt layers; it was replaced by the present caisson-based structure in the mid-'60's, a few years after the original I-10 and I-15 (now 215) bridges were built.
I have yet to see pictures of any US 66/US 99 co-signage along Foothill Blvd. in the years between 1926 and 1931; if Dan Faigin or others have any photographs showing this, please post; it will be greatly appreciated! Actually, anything showing reassurance or trailblazer signage from the era and region would really be a gift.
I'll take an educated guess that the Division of Highways always intended any lengthy 66/99 multiplex in the region to be a temporary arrangement until a more direct through route into Los Angeles could be established for US 99.