I got my notification today from the City, and the nice lady who's officially a neutral examiner has upheld my citation.
Et tu, Bruté?
That means the City keeps my $55, and I can fight it in court if I so choose.
The only reason I can think of (well, two reasons) is that the examiner thinks I failed to present a "preponderance of evidence" in support of my position. The other possibility is that the City's official records have been tampered with (or filed falsely from the start) to show an installation date for the sign that pre-dated my parking ticket.
If I go to court, it's a $25 filing fee, and I have no idea how it all works and whether I'll need an attorney and all the rest. I haven't exactly been living my life in and out of court.
But I did stop at the neighbors' house on the corner where the sign is. They speak Spanish, but we managed to communicate well enough for them to tell me that they went on vacation in January. When they left, there was no sign. When they got back, the sign was there. Enero. No doubt about it.
Next, I'll try to find out what the City has on record as their sign-installation date, and then I'll go from there.