Hi David.  I believe that nobody pays much attention to front plates 
anymore.  However, I think that you  only have to display a front 
plate if the place of registration requires one.  ie. BC vehicles 
require front plates, and if you are missing it, you can get a ticket 
if the police are bored.  If an Alberta vehicle, which doesn't 
require a front plate, is LEGAL in BC since the origin 
of registration doesn't require it.

Now, if California does require a front plate, then I would think 
that you would be legally bound to have it on your vehicle.

The chances of getting stopped for just a front plate would be pretty 
slim in my opinion, you would  have to get a hauled over for 
something else... speeding !!.... for them to look for other things.

Hope this helps.

Harry.  
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Harry Yates      Internet: hyates@bcit.bc.ca  Burnaby, BC
'90 Syncro Westfalia
'87 Golf Diesel