A security guard walking down US Highway 101 where there are towering stacks of hollow iron floats from which the iron antisubmarine nets were suspended to protect the US ports during WWII

Yes I looked at a 1955 map of Seal Beach, where it's labeled as The Pacific Coast Highway with no number given. But it's CA-1, not Interstate 1, that's on the East Coast. The 1 and the 101 do run together in several places, let me go to Wikipedia for some history.

Okay! Here's a relevant paragraph after a whole section about building and renaming parts of what's now CA-1:

"The state Legislature in 1963 tossed out the old conflicting Legislative Route Numbers (1964 renumbering), got rid of some famous old U.S. routes, and renumbered many state highways. It abolished US 101A in Los Angeles, Orange and Ventura counties and renumbered it as SR 1. The Rockport to Leggett connection then became State Route 208.[51] The cover of California Highways magazine for in early 1964 shows state engineers posting the new shield at Point Mugu.[52] The same year, the Legislature by state law named SR 1 "Pacific Coast Highway" in Orange, Los Angeles and Ventura counties, "Cabrillo Highway" from Santa Barbara north to San Francisco, and "Shoreline Highway" from Marin County to its northern terminus. Many cities, however, did not change the name of city streets that are part of SR 1, such as Lincoln and Sepulveda boulevards in Los Angeles, Santa Monica and El Segundo; and Junipero Serra and Park Presidio boulevards in San Francisco. Several other cities and communities like Newport Beach, Laguna Beach and Bodega Bay merely named their respective city streets as "Coast Highway".

So back in 1953, PCH down in Seal Beach might have been part of old 101-A!