Paper or Plastic? Spade or Shield?

Interstate 880 at 23rd Avenue in Oakland

Is it an Interstate or a state highway? This sign on the 23rd Avenue overpass over Interstate 880 in Oakland seems to answer, "yes". Probably the sign was originally made for the freeway's original designation as California 17. When it became I-880, the Interstate shield just covered up the 17 on the original marker. That's just speculation on my part, especially considering that the California spade outline seems unusually wide for containing the number 17. So there is the remote possibility that this might be a sign goof. In any event, the condition of the button reflectors on the 880 shield indicate that this sign has been around for many years.