When I began searching out the alignments of Route 66 in St. Louis in 1978, every bridge across the Mississippi seemed to have carried the old road.
In 1926 when the highway was designated the old McKinley Bridge, that bridge that carried both cars and trains on the same level, first carried Route 66 across the Mississippi to Salisbury Street From there the highway went out Natural Bridge to Prairie and crossed Fairgrounds Park to Vanderventer. Vanderventer carried it to Clayton Avenue, which took it through Forest Park to Skinker at HiPoint. Skinker bled into McCausland, which carried it to Manchester Road and out to Pond, Missouri, where it hitched up with the Old State Road, Missouri 100, which is the only road going west from St. Louis that does not cross a river.
Rick Dinkela has Route 66 crossing the MacArthur Bridge, also known as the free bridge, and going west along Chouteau to Manchester and out the state road in 1927
The Scott brothers started planning the Chain of Rocks Bridge at the extreme northern end of St. Louis and completed the Chain of Rocks Bridge in 1929, but neither Missouri nor Illinois put the bridge on their official maps. The brothers paved an additional 600 feet of roadway to connect their bridge to Lindberg Blvd., which became the 66-Bypass, but it was a long time before the bridge carried 66 across the river officially.
The brothers sold the bridge to Madison, Illinois, which got it on the official maps and turned it into a golden goose for the city.
When it did, the Chain of Rocks Bridge dumped travelers out onto Riverview Blvd., which carried people to West Florissant, which bled into Twelfth Street. That alignment probably hitched up with the Chouteau-Manchester alignment until 1933, when the State of Missouri moved Route 66 off the ridge road that carried the Old State Road and moved it down into the Meramec Valley and built a new Route 66 that picked up the Old State Road at Grey Summit.
After that Twelfth Street, which became Gravois Blvd, which became Chippewa, which became Watson carried the road out to the new Route 66 in the Meramec Valley. This was City 66 and crossed every major north-south-east-west street in St. Louis.
So after 1933, City 66 followed West Florissant, Twelfth, etc; Lindberg was the 66 Bypass to Watson; Kingshighway, which ends at West Florissant, was also a 66 Bypass and carried the traveler to Chippewa and west.