This is my first post so I hope the attachment works. This photo from the May 1925 edition of California Highways and Public Works should clear things up. The original crossing of the San Joaquin River was east of the 1926 crossing.It appears that it was probably close to where the I-5 bridges are now. It then went under the wooden trestle in your second photo. That short strip of concrete is probably the original roadbed. A very cool find. I live in Manteca and will be going to check it out tomorrow.