Calif glider property tax fishing?

Last week I received an official looking letter from the Assessor for
Kern County (Bakersfield), demanding to know the location and value of
my glider on 1 Jan 05. It threatens to impose a penalty if I fail to
file the enclosed form by 7 May.

The puzzling thing is, my glider has never been in or near that county.
I'm thinking of calling them (no toll free number provided), and
inquiring as to why they think I owe them a form at all.

Is this just a fishing expedition on their part? Can I just ignore it?
Can they be required to prove my glider was ever in Kern County?

If you've been through this, you may reply privately if you wish.

Thanks,
Ken