The Manhattan Mercury: October 22 1978
BH Photo #457428
I did think about the Keystone plaque but they flaired out and were a little wider at the bottom than WIBCo ones.
BTW, I think you are right, but thought I’d double check.
I have seen a bridge plaque with that shape with “Keystone” on it :^)
Given the 1900 build date, Are you basing the builder by the plaque shape or secondary evidence?