Notice the photo credit: Mayor Eric Garcetti is the photographer and author of the commemorative message printed on the back of the plate.
"The Sixth Street bridge has stood as a testament to the vision and determination of the men and women who built Los Angeles, and has strengthened our city by connecting the diverse neighborhoods to the east and west of the L.A. River. As we build a modern landmark to meet the needs of our growing community, this contemporary structure will also serve as a link between our city's history and future --- and will play a pivotal role in shaping our City's identity in the 21st century." - Mayor Eric Garcetti
The design motif on the back of the plate was inspired by details in the architecture of the Sixth Street Bridge itself, as can be seen in photo of the bridge below.